Merge pull request #15606 from senny/ar/foreign_key_support

Basic support for adding and removing foreign keys

20 files changed, 739 insertions, 70 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 4b39791b73..9a9c0367e1 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,15 @@
+*   Support for adding and removing foreign keys. Foreign keys are now
+    a part of `schema.rb`. This is supported by Mysql2Adapter, MysqlAdapter
+    and PostgreSQLAdapter.
+
+    Example:
+
+        # within your migrations:
+        add_foreign_key :articles, :authors
+        remove_foreign_key :articles, :authors
+
+    *Yves Senn*
+
 *   Fix subtle bugs regarding attribute assignment on models with no primary
     key. `'id'` will no longer be part of the attributes hash.
 
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
index 47fe501752..c1379f6bec 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_creation.rb
@@ -23,6 +23,8 @@ module ActiveRecord
           def visit_AlterTable(o)
             sql = "ALTER TABLE #{quote_table_name(o.name)} "
             sql << o.adds.map { |col| visit_AddColumn col }.join(' ')
+            sql << o.foreign_key_adds.map { |fk| visit_AddForeignKey fk }.join(' ')
+            sql << o.foreign_key_drops.map { |fk| visit_DropForeignKey fk }.join(' ')
           end
 
           def visit_ColumnDefinition(o)
@@ -41,6 +43,21 @@ module ActiveRecord
             create_sql
           end
 
+          def visit_AddForeignKey(o)
+            sql = <<-SQL.strip_heredoc
+              ADD CONSTRAINT #{quote_column_name(o.name)}
+              FOREIGN KEY (#{quote_column_name(o.column)})
+                REFERENCES #{quote_table_name(o.to_table)} (#{quote_column_name(o.primary_key)})
+            SQL
+            sql << " #{action_sql('DELETE', o.on_delete)}" if o.on_delete
+            sql << " #{action_sql('UPDATE', o.on_update)}" if o.on_update
+            sql
+          end
+
+          def visit_DropForeignKey(name)
+            "DROP CONSTRAINT #{quote_column_name(name)}"
+          end
+
           def column_options(o)
             column_options = {}
             column_options[:null] = o.null unless o.null.nil?
@@ -84,6 +101,19 @@ module ActiveRecord
           def options_include_default?(options)
             options.include?(:default) && !(options[:null] == false && options[:default].nil?)
           end
+
+          def action_sql(action, dependency)
+            case dependency
+            when :nullify then "ON #{action} SET NULL"
+            when :cascade  then "ON #{action} CASCADE"
+            when :restrict then "ON #{action} RESTRICT"
+            else
+              raise ArgumentError, <<-MSG.strip_heredoc
+                '#{dependency}' is not supported for :on_update or :on_delete.
+                Supported values are: :nullify, :cascade, :restrict
+              MSG
+            end
+          end
       end
     end
   end
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
index a9b3e9cfb9..785cfd9dbc 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb
@@ -25,6 +25,37 @@ module ActiveRecord
     class ChangeColumnDefinition < Struct.new(:column, :type, :options) #:nodoc:
     end
 
+    class ForeignKeyDefinition < Struct.new(:from_table, :to_table, :options)
+      def name
+        options[:name]
+      end
+
+      def column
+        options[:column]
+      end
+
+      def primary_key
+        options[:primary_key] || default_primary_key
+      end
+
+      def on_delete
+        options[:on_delete]
+      end
+
+      def on_update
+        options[:on_update]
+      end
+
+      def custom_primary_key?
+        options[:primary_key] != default_primary_key
+      end
+
+      private
+      def default_primary_key
+        "id"
+      end
+    end
+
     # Represents the schema of an SQL table in an abstract way. This class
     # provides methods for manipulating the schema representation.
     #
@@ -303,14 +334,26 @@ module ActiveRecord
 
     class AlterTable # :nodoc:
       attr_reader :adds
+      attr_reader :foreign_key_adds
+      attr_reader :foreign_key_drops
 
       def initialize(td)
         @td   = td
         @adds = []
+        @foreign_key_adds = []
+        @foreign_key_drops = []
       end
 
       def name; @td.name; end
 
+      def add_foreign_key(to_table, options)
+        @foreign_key_adds << ForeignKeyDefinition.new(name, to_table, options)
+      end
+
+      def drop_foreign_key(name)
+        @foreign_key_drops << name
+      end
+
       def add_column(name, type, options)
         name = name.to_s
         type = type.to_sym
diff --git a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
index 22823a8c58..5814c2b711 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb
@@ -642,6 +642,115 @@ module ActiveRecord
       end
       alias :remove_belongs_to :remove_reference
 
+      # Returns an array of foreign keys for the given table.
+      # The foreign keys are represented as +ForeignKeyDefinition+ objects.
+      def foreign_keys(table_name)
+        raise NotImplementedError, "foreign_keys is not implemented"
+      end
+
+      # Adds a new foreign key. +from_table+ is the table with the key column,
+      # +to_table+ contains the referenced primary key.
+      #
+      # The foreign key will be named after the following pattern: <tt>fk_rails_<identifier></tt>.
+      # +identifier+ is a 10 character long random string. A custom name can be specified with
+      # the <tt>:name</tt> option.
+      #
+      # ====== Creating a simple foreign key
+      #
+      #   add_foreign_key :articles, :authors
+      #
+      # generates:
+      #
+      #   ALTER TABLE "articles" ADD CONSTRAINT articles_author_id_fk FOREIGN KEY ("author_id") REFERENCES "authors" ("id")
+      #
+      # ====== Creating a foreign key on a specific column
+      #
+      #   add_foreign_key :articles, :users, column: :author_id, primary_key: "lng_id"
+      #
+      # generates:
+      #
+      #   ALTER TABLE "articles" ADD CONSTRAINT fk_rails_58ca3d3a82 FOREIGN KEY ("author_id") REFERENCES "users" ("lng_id")
+      #
+      # ====== Creating a cascading foreign key
+      #
+      #   add_foreign_key :articles, :authors, on_delete: :cascade
+      #
+      # generates:
+      #
+      #   ALTER TABLE "articles" ADD CONSTRAINT articles_author_id_fk FOREIGN KEY ("author_id") REFERENCES "authors" ("id") ON DELETE CASCADE
+      #
+      # The +options+ hash can include the following keys:
+      # [<tt>:column</tt>]
+      #   The foreign key column name on +from_table+. Defaults to <tt>to_table.singularize + "_id"</tt>
+      # [<tt>:primary_key</tt>]
+      #   The primary key column name on +to_table+. Defaults to +id+.
+      # [<tt>:name</tt>]
+      #   The constraint name. Defaults to <tt>fk_rails_<identifier></tt>.
+      # [<tt>:on_delete</tt>]
+      #   Action that happens <tt>ON DELETE</tt>. Valid values are +:nullify+, +:cascade:+ and +:restrict+
+      # [<tt>:on_update</tt>]
+      #   Action that happens <tt>ON UPDATE</tt>. Valid values are +:nullify+, +:cascade:+ and +:restrict+
+      def add_foreign_key(from_table, to_table, options = {})
+        return unless supports_foreign_keys?
+
+        options[:column] ||= foreign_key_column_for(to_table)
+
+        options = {
+          column: options[:column],
+          primary_key: options[:primary_key],
+          name: foreign_key_name(from_table, options),
+          on_delete: options[:on_delete],
+          on_update: options[:on_update]
+        }
+        at = create_alter_table from_table
+        at.add_foreign_key to_table, options
+
+        execute schema_creation.accept(at)
+      end
+
+      # Removes the given foreign key from the table.
+      #
+      # Removes the foreign key on +accounts.branch_id+.
+      #
+      #   remove_foreign_key :accounts, :branches
+      #
+      # Removes the foreign key on +accounts.owner_id+.
+      #
+      #   remove_foreign_key :accounts, column: :owner_id
+      #
+      # Removes the foreign key named +special_fk_name+ on the +accounts+ table.
+      #
+      #   remove_foreign_key :accounts, name: :special_fk_name
+      #
+      def remove_foreign_key(from_table, options_or_to_table = {})
+        return unless supports_foreign_keys?
+
+        if options_or_to_table.is_a?(Hash)
+          options = options_or_to_table
+        else
+          options = { column: foreign_key_column_for(options_or_to_table) }
+        end
+
+        fk_name_to_delete = options.fetch(:name) do
+          fk_to_delete = foreign_keys(from_table).detect {|fk| fk.column == options[:column] }
+
+          if fk_to_delete
+            fk_to_delete.name
+          else
+            raise ArgumentError, "Table '#{from_table}' has no foreign key on column '#{options[:column]}'"
+          end
+        end
+
+        at = create_alter_table from_table
+        at.drop_foreign_key fk_name_to_delete
+
+        execute schema_creation.accept(at)
+      end
+
+      def foreign_key_column_for(table_name) # :nodoc:
+        "#{table_name.to_s.singularize}_id"
+      end
+
       def dump_schema_information #:nodoc:
         sm_table = ActiveRecord::Migrator.schema_migrations_table_name
 
@@ -852,6 +961,12 @@ module ActiveRecord
       def create_alter_table(name)
         AlterTable.new create_table_definition(name, false, {})
       end
+
+      def foreign_key_name(table_name, options) # :nodoc:
+        options.fetch(:name) do
+          "fk_rails_#{SecureRandom.hex(5)}"
+        end
+      end
     end
   end
 end
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
index cc494a7f40..294ed6d7bf 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb
@@ -233,6 +233,11 @@ module ActiveRecord
         false
       end
 
+      # Does this adapter support creating foreign key constraints?
+      def supports_foreign_keys?
+        false
+      end
+
       # This is meant to be implemented by the adapters that support extensions
       def disable_extension(name)
       end
diff --git a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
index def04dbed2..4924f345fc 100644
--- a/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb
@@ -10,6 +10,10 @@ module ActiveRecord
           add_column_position!(super, column_options(o))
         end
 
+        def visit_DropForeignKey(name)
+          "DROP FOREIGN KEY #{name}"
+        end
+
         private
 
         def visit_TableDefinition(o)
@@ -192,6 +196,10 @@ module ActiveRecord
         true
       end
 
+      def supports_foreign_keys?
+        true
+      end
+
       def native_database_types
         NATIVE_DATABASE_TYPES
       end
@@ -501,6 +509,34 @@ module ActiveRecord
         execute "CREATE #{index_type} INDEX #{quote_column_name(index_name)} #{index_using} ON #{quote_table_name(table_name)} (#{index_columns})#{index_options} #{index_algorithm}"
       end
 
+      def foreign_keys(table_name)
+        fk_info = select_all <<-SQL.strip_heredoc
+          SELECT fk.referenced_table_name as 'to_table'
+                ,fk.referenced_column_name as 'primary_key'
+                ,fk.column_name as 'column'
+                ,fk.constraint_name as 'name'
+          FROM information_schema.key_column_usage fk
+          WHERE fk.referenced_column_name is not null
+            AND fk.table_schema = '#{@config[:database]}'
+            AND fk.table_name = '#{table_name}'
+        SQL
+
+        create_table_info = select_one("SHOW CREATE TABLE #{quote_table_name(table_name)}")["Create Table"]
+
+        fk_info.map do |row|
+          options = {
+            column: row['column'],
+            name: row['name'],
+            primary_key: row['primary_key']
+          }
+
+          options[:on_update] = extract_foreign_key_action(create_table_info, row['name'], "UPDATE")
+          options[:on_delete] = extract_foreign_key_action(create_table_info, row['name'], "DELETE")
+
+          ForeignKeyDefinition.new(table_name, row['to_table'], options)
+        end
+      end
+
       # Maps logical Rails types to MySQL-specific data types.
       def type_to_sql(type, limit = nil, precision = nil, scale = nil)
         case type.to_s
@@ -779,6 +815,15 @@ module ActiveRecord
         # ...and send them all in one query
         @connection.query  "SET #{encoding} #{variable_assignments}"
       end
+
+      def extract_foreign_key_action(structure, name, action) # :nodoc:
+        if structure =~ /CONSTRAINT #{quote_column_name(name)} FOREIGN KEY .* REFERENCES .* ON #{action} (CASCADE|SET NULL|RESTRICT)/
+          case $1
+          when 'CASCADE'; :cascade
+          when 'SET NULL'; :nullify
+          end
+        end
+      end
     end
   end
 end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
index b2aeb3a058..30df98be1b 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb
@@ -448,6 +448,42 @@ module ActiveRecord
           execute "ALTER INDEX #{quote_column_name(old_name)} RENAME TO #{quote_table_name(new_name)}"
         end
 
+        def foreign_keys(table_name)
+          fk_info = select_all <<-SQL.strip_heredoc
+            SELECT t2.relname AS to_table, a1.attname AS column, a2.attname AS primary_key, c.conname AS name, c.confupdtype AS on_update, c.confdeltype AS on_delete
+            FROM pg_constraint c
+            JOIN pg_class t1 ON c.conrelid = t1.oid
+            JOIN pg_class t2 ON c.confrelid = t2.oid
+            JOIN pg_attribute a1 ON a1.attnum = c.conkey[1] AND a1.attrelid = t1.oid
+            JOIN pg_attribute a2 ON a2.attnum = c.confkey[1] AND a2.attrelid = t2.oid
+            JOIN pg_namespace t3 ON c.connamespace = t3.oid
+            WHERE c.contype = 'f'
+              AND t1.relname = #{quote(table_name)}
+              AND t3.nspname = ANY (current_schemas(false))
+            ORDER BY c.conname
+          SQL
+
+          fk_info.map do |row|
+            options = {
+              column: row['column'],
+              name: row['name'],
+              primary_key: row['primary_key']
+            }
+
+            options[:on_delete] = extract_foreign_key_action(row['on_delete'])
+            options[:on_update] = extract_foreign_key_action(row['on_update'])
+            ForeignKeyDefinition.new(table_name, row['to_table'], options)
+          end
+        end
+
+        def extract_foreign_key_action(specifier) # :nodoc:
+          case specifier
+          when 'c'; :cascade
+          when 'n'; :nullify
+          when 'r'; :restrict
+          end
+        end
+
         def index_name_length
           63
         end
diff --git a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
index be4ae47d09..34262cf91d 100644
--- a/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
+++ b/activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
@@ -159,6 +159,10 @@ module ActiveRecord
         true
       end
 
+      def supports_foreign_keys?
+        true
+      end
+
       def index_algorithms
         { concurrently: 'CONCURRENTLY' }
       end
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index 01c001e692..12eaf36156 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -645,7 +645,9 @@ module ActiveRecord
         unless @connection.respond_to? :revert
           unless arguments.empty? || [:execute, :enable_extension, :disable_extension].include?(method)
             arguments[0] = proper_table_name(arguments.first, table_name_options)
-            arguments[1] = proper_table_name(arguments.second, table_name_options) if method == :rename_table
+            if [:rename_table, :add_foreign_key].include?(method)
+              arguments[1] = proper_table_name(arguments.second, table_name_options)
+            end
           end
         end
         return super unless connection.respond_to?(method)
diff --git a/activerecord/lib/active_record/migration/command_recorder.rb b/activerecord/lib/active_record/migration/command_recorder.rb
index c44d8c1665..f833caaab6 100644
--- a/activerecord/lib/active_record/migration/command_recorder.rb
+++ b/activerecord/lib/active_record/migration/command_recorder.rb
@@ -74,7 +74,9 @@ module ActiveRecord
         :rename_index, :rename_column, :add_index, :remove_index, :add_timestamps, :remove_timestamps,
         :change_column_default, :add_reference, :remove_reference, :transaction,
         :drop_join_table, :drop_table, :execute_block, :enable_extension,
-        :change_column, :execute, :remove_columns, :change_column_null # irreversible methods need to be here too
+        :change_column, :execute, :remove_columns, :change_column_null,
+        :add_foreign_key, :remove_foreign_key
+       # irreversible methods need to be here too
       ].each do |method|
         class_eval <<-EOV, __FILE__, __LINE__ + 1
           def #{method}(*args, &block)          # def create_table(*args, &block)
@@ -167,6 +169,21 @@ module ActiveRecord
         [:change_column_null, args]
       end
 
+      def invert_add_foreign_key(args)
+        from_table, to_table, add_options = args
+        add_options ||= {}
+
+        if add_options[:name]
+          options = { name: add_options[:name] }
+        elsif add_options[:column]
+          options = { column: add_options[:column] }
+        else
+          options = to_table
+        end
+
+        [:remove_foreign_key, [from_table, options]]
+      end
+
       # Forwards any missing method call to the \target.
       def method_missing(method, *args, &block)
         if @delegate.respond_to?(method)
diff --git a/activerecord/lib/active_record/schema_dumper.rb b/activerecord/lib/active_record/schema_dumper.rb
index e055d571ab..64bc68eefd 100644
--- a/activerecord/lib/active_record/schema_dumper.rb
+++ b/activerecord/lib/active_record/schema_dumper.rb
@@ -91,7 +91,8 @@ HEADER
       end
 
       def tables(stream)
-        @connection.tables.sort.each do |tbl|
+        sorted_tables = @connection.tables.sort
+        sorted_tables.each do |tbl|
           next if ['schema_migrations', ignore_tables].flatten.any? do |ignored|
             case ignored
             when String; remove_prefix_and_suffix(tbl) == ignored
@@ -102,6 +103,13 @@ HEADER
           end
           table(tbl, stream)
         end
+
+        # dump foreign keys at the end to make sure all dependent tables exist.
+        if @connection.supports_foreign_keys?
+          sorted_tables.each do |tbl|
+            foreign_keys(tbl, stream)
+          end
+        end
       end
 
       def table(table, stream)
@@ -212,6 +220,36 @@ HEADER
         end
       end
 
+      def foreign_keys(table, stream)
+        if (foreign_keys = @connection.foreign_keys(table)).any?
+          add_foreign_key_statements = foreign_keys.map do |foreign_key|
+            parts = [
+                     'add_foreign_key ' + remove_prefix_and_suffix(foreign_key.from_table).inspect,
+                     remove_prefix_and_suffix(foreign_key.to_table).inspect,
+                    ]
+
+            if foreign_key.column != @connection.foreign_key_column_for(foreign_key.to_table)
+              parts << ('column: ' + foreign_key.column.inspect)
+            end
+
+            if foreign_key.custom_primary_key?
+              parts << ('primary_key: ' + foreign_key.primary_key.inspect)
+            end
+
+            if foreign_key.name !~ /^fk_rails_[0-9a-f]{10}$/
+              parts << ('name: ' + foreign_key.name.inspect)
+            end
+
+            parts << ('on_update: ' + foreign_key.on_update.inspect) if foreign_key.on_update
+            parts << ('on_delete: ' + foreign_key.on_delete.inspect) if foreign_key.on_delete
+
+            '  ' + parts.join(', ')
+          end
+
+          stream.puts add_foreign_key_statements.sort.join("\n")
+        end
+      end
+
       def remove_prefix_and_suffix(table)
         table.gsub(/^(#{@options[:table_name_prefix]})(.+)(#{@options[:table_name_suffix]})$/,  "\\2")
       end
diff --git a/activerecord/test/cases/migration/command_recorder_test.rb b/activerecord/test/cases/migration/command_recorder_test.rb
index 1c0134843b..e955beae1a 100644
--- a/activerecord/test/cases/migration/command_recorder_test.rb
+++ b/activerecord/test/cases/migration/command_recorder_test.rb
@@ -270,6 +270,31 @@ module ActiveRecord
         enable = @recorder.inverse_of :disable_extension, ['uuid-ossp']
         assert_equal [:enable_extension, ['uuid-ossp'], nil], enable
       end
+
+      def test_invert_add_foreign_key
+        enable = @recorder.inverse_of :add_foreign_key, [:dogs, :people]
+        assert_equal [:remove_foreign_key, [:dogs, :people]], enable
+      end
+
+      def test_invert_add_foreign_key_with_column
+        enable = @recorder.inverse_of :add_foreign_key, [:dogs, :people, column: "owner_id"]
+        assert_equal [:remove_foreign_key, [:dogs, column: "owner_id"]], enable
+      end
+
+      def test_invert_add_foreign_key_with_column_and_name
+        enable = @recorder.inverse_of :add_foreign_key, [:dogs, :people, column: "owner_id", name: "fk"]
+        assert_equal [:remove_foreign_key, [:dogs, name: "fk"]], enable
+      end
+
+      def test_remove_foreign_key_is_irreversible
+        assert_raises ActiveRecord::IrreversibleMigration do
+          @recorder.inverse_of :remove_foreign_key, [:dogs, column: "owner_id"]
+        end
+
+        assert_raises ActiveRecord::IrreversibleMigration do
+          @recorder.inverse_of :remove_foreign_key, [:dogs, name: "fk"]
+        end
+      end
     end
   end
 end
diff --git a/activerecord/test/cases/migration/foreign_key_test.rb b/activerecord/test/cases/migration/foreign_key_test.rb
new file mode 100644
index 0000000000..c985092b4c
--- /dev/null
+++ b/activerecord/test/cases/migration/foreign_key_test.rb
@@ -0,0 +1,242 @@
+require 'cases/helper'
+require 'support/ddl_helper'
+require 'support/schema_dumping_helper'
+
+if ActiveRecord::Base.connection.supports_foreign_keys?
+module ActiveRecord
+  class Migration
+    class ForeignKeyTest < ActiveRecord::TestCase
+      include DdlHelper
+      include SchemaDumpingHelper
+
+      class Rocket < ActiveRecord::Base
+      end
+
+      class Astronaut < ActiveRecord::Base
+      end
+
+      setup do
+        @connection = ActiveRecord::Base.connection
+        @connection.create_table "rockets" do |t|
+          t.string :name
+        end
+
+        @connection.create_table "astronauts" do |t|
+          t.string :name
+          t.references :rocket
+        end
+      end
+
+      teardown do
+        if defined?(@connection)
+          @connection.execute "DROP TABLE IF EXISTS astronauts"
+          @connection.execute "DROP TABLE IF EXISTS rockets"
+        end
+      end
+
+      def test_foreign_keys
+        foreign_keys = @connection.foreign_keys("fk_test_has_fk")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal "fk_test_has_fk", fk.from_table
+        assert_equal "fk_test_has_pk", fk.to_table
+        assert_equal "fk_id", fk.column
+        assert_equal "pk_id", fk.primary_key
+        assert_equal "fk_name", fk.name
+      end
+
+      def test_add_foreign_key_inferes_column
+        @connection.add_foreign_key :astronauts, :rockets
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal "astronauts", fk.from_table
+        assert_equal "rockets", fk.to_table
+        assert_equal "rocket_id", fk.column
+        assert_equal "id", fk.primary_key
+        assert_match(/^fk_rails_.{10}$/, fk.name)
+      end
+
+      def test_add_foreign_key_with_column
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id"
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal "astronauts", fk.from_table
+        assert_equal "rockets", fk.to_table
+        assert_equal "rocket_id", fk.column
+        assert_equal "id", fk.primary_key
+        assert_match(/^fk_rails_.{10}$/, fk.name)
+      end
+
+      def test_add_foreign_key_with_non_standard_primary_key
+        with_example_table @connection, "space_shuttles", "pk integer PRIMARY KEY" do
+          @connection.add_foreign_key(:astronauts, :space_shuttles,
+                                      column: "rocket_id", primary_key: "pk", name: "custom_pk")
+
+          foreign_keys = @connection.foreign_keys("astronauts")
+          assert_equal 1, foreign_keys.size
+
+          fk = foreign_keys.first
+          assert_equal "astronauts", fk.from_table
+          assert_equal "space_shuttles", fk.to_table
+          assert_equal "pk", fk.primary_key
+
+          @connection.remove_foreign_key :astronauts, name: "custom_pk"
+        end
+      end
+
+      def test_add_on_delete_restrict_foreign_key
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :restrict
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        if current_adapter?(:MysqlAdapter, :Mysql2Adapter)
+          # ON DELETE RESTRICT is the default on MySQL
+          assert_equal nil, fk.on_delete
+        else
+          assert_equal :restrict, fk.on_delete
+        end
+      end
+
+      def test_add_on_delete_cascade_foreign_key
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :cascade
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal :cascade, fk.on_delete
+      end
+
+      def test_add_on_delete_nullify_foreign_key
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :nullify
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal :nullify, fk.on_delete
+      end
+
+      def test_on_update_and_on_delete_raises_with_invalid_values
+        assert_raises ArgumentError do
+          @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :invalid
+        end
+
+        assert_raises ArgumentError do
+          @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_update: :invalid
+        end
+      end
+
+      def test_add_foreign_key_with_on_update
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_update: :nullify
+
+        foreign_keys = @connection.foreign_keys("astronauts")
+        assert_equal 1, foreign_keys.size
+
+        fk = foreign_keys.first
+        assert_equal :nullify, fk.on_update
+      end
+
+      def test_remove_foreign_key_inferes_column
+        @connection.add_foreign_key :astronauts, :rockets
+
+        assert_equal 1, @connection.foreign_keys("astronauts").size
+        @connection.remove_foreign_key :astronauts, :rockets
+        assert_equal [], @connection.foreign_keys("astronauts")
+      end
+
+      def test_remove_foreign_key_by_column
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id"
+
+        assert_equal 1, @connection.foreign_keys("astronauts").size
+        @connection.remove_foreign_key :astronauts, column: "rocket_id"
+        assert_equal [], @connection.foreign_keys("astronauts")
+      end
+
+      def test_remove_foreign_key_by_name
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", name: "fancy_named_fk"
+
+        assert_equal 1, @connection.foreign_keys("astronauts").size
+        @connection.remove_foreign_key :astronauts, name: "fancy_named_fk"
+        assert_equal [], @connection.foreign_keys("astronauts")
+      end
+
+      def test_remove_foreign_non_existing_foreign_key_raises
+        assert_raises ArgumentError do
+          @connection.remove_foreign_key :astronauts, :rockets
+        end
+      end
+
+      def test_schema_dumping
+        @connection.add_foreign_key :astronauts, :rockets
+        output = dump_table_schema "astronauts"
+        assert_match %r{\s+add_foreign_key "astronauts", "rockets"$}, output
+      end
+
+      def test_schema_dumping_with_options
+        output = dump_table_schema "fk_test_has_fk"
+        assert_match %r{\s+add_foreign_key "fk_test_has_fk", "fk_test_has_pk", column: "fk_id", primary_key: "pk_id", name: "fk_name"$}, output
+      end
+
+      def test_schema_dumping_on_delete_and_on_update_options
+        @connection.add_foreign_key :astronauts, :rockets, column: "rocket_id", on_delete: :nullify, on_update: :cascade
+
+        output = dump_table_schema "astronauts"
+        assert_match %r{\s+add_foreign_key "astronauts",.+on_update: :cascade,.+on_delete: :nullify$}, output
+      end
+
+      class CreateCitiesAndHousesMigration < ActiveRecord::Migration
+        def change
+          create_table("cities") { |t| }
+
+          create_table("houses") do |t|
+            t.column :city_id, :integer
+          end
+          add_foreign_key :houses, :cities, column: "city_id"
+        end
+      end
+
+      def test_add_foreign_key_is_reversible
+        migration = CreateCitiesAndHousesMigration.new
+        silence_stream($stdout) { migration.migrate(:up) }
+        assert_equal 1, @connection.foreign_keys("houses").size
+      ensure
+        silence_stream($stdout) { migration.migrate(:down) }
+      end
+    end
+  end
+end
+else
+module ActiveRecord
+  class Migration
+    class NoForeignKeySupportTest < ActiveRecord::TestCase
+      setup do
+        @connection = ActiveRecord::Base.connection
+      end
+
+      def test_add_foreign_key_should_be_noop
+        @connection.add_foreign_key :clubs, :categories
+      end
+
+      def test_remove_foreign_key_should_be_noop
+        @connection.remove_foreign_key :clubs, :categories
+      end
+
+      def test_foreign_keys_should_raise_not_implemented
+        assert_raises NotImplementedError do
+          @connection.foreign_keys("clubs")
+        end
+      end
+    end
+  end
+end
+end
diff --git a/activerecord/test/cases/schema_dumper_test.rb b/activerecord/test/cases/schema_dumper_test.rb
index 5f02d39e32..4e71d04bc0 100644
--- a/activerecord/test/cases/schema_dumper_test.rb
+++ b/activerecord/test/cases/schema_dumper_test.rb
@@ -372,15 +372,28 @@ class SchemaDumperTest < ActiveRecord::TestCase
     assert_match %r{create_table "subscribers", id: false}, output
   end
 
+  if ActiveRecord::Base.connection.supports_foreign_keys?
+    def test_foreign_keys_are_dumped_at_the_bottom_to_circumvent_dependency_issues
+      output = standard_dump
+      assert_match(/^\s+add_foreign_key "fk_test_has_fk"[^\n]+\n\s+add_foreign_key "lessons_students"/, output)
+    end
+  end
+
   class CreateDogMigration < ActiveRecord::Migration
     def up
+      create_table("dog_owners") do |t|
+      end
+
       create_table("dogs") do |t|
         t.column :name, :string
+        t.column :owner_id, :integer
       end
       add_index "dogs", [:name]
+      add_foreign_key :dogs, :dog_owners, column: "owner_id" if supports_foreign_keys?
     end
     def down
       drop_table("dogs")
+      drop_table("dog_owners")
     end
   end
 
@@ -396,13 +409,17 @@ class SchemaDumperTest < ActiveRecord::TestCase
     assert_no_match %r{create_table "foo_.+_bar"}, output
     assert_no_match %r{add_index "foo_.+_bar"}, output
     assert_no_match %r{create_table "schema_migrations"}, output
+
+    if ActiveRecord::Base.connection.supports_foreign_keys?
+      assert_no_match %r{add_foreign_key "foo_.+_bar"}, output
+      assert_no_match %r{add_foreign_key "[^"]+", "foo_.+_bar"}, output
+    end
   ensure
     migration.migrate(:down)
 
     ActiveRecord::Base.table_name_suffix = ActiveRecord::Base.table_name_prefix = ''
     $stdout = original
   end
-
 end
 
 class SchemaDumperDefaultsTest < ActiveRecord::TestCase
diff --git a/activerecord/test/fixtures/fk_test_has_pk.yml b/activerecord/test/fixtures/fk_test_has_pk.yml
index c93952180b..73882bac41 100644
--- a/activerecord/test/fixtures/fk_test_has_pk.yml
+++ b/activerecord/test/fixtures/fk_test_has_pk.yml
@@ -1,2 +1,2 @@
 first:
-  id: 1
\ No newline at end of file
+  pk_id: 1
\ No newline at end of file
diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 6ce82c71a8..03d33c151a 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -855,10 +855,10 @@ ActiveRecord::Schema.define do
       t.integer :fk_id, null: false
     end
 
-    create_table :fk_test_has_pk, force: true do |t|
+    create_table :fk_test_has_pk, force: true, primary_key: "pk_id" do |t|
     end
 
-    execute "ALTER TABLE fk_test_has_fk ADD CONSTRAINT fk_name FOREIGN KEY (#{quote_column_name 'fk_id'}) REFERENCES #{quote_table_name 'fk_test_has_pk'} (#{quote_column_name 'id'})"
+    execute "ALTER TABLE fk_test_has_fk ADD CONSTRAINT fk_name FOREIGN KEY (#{quote_column_name 'fk_id'}) REFERENCES #{quote_table_name 'fk_test_has_pk'} (#{quote_column_name 'pk_id'})"
 
     execute "ALTER TABLE lessons_students ADD CONSTRAINT student_id_fk FOREIGN KEY (#{quote_column_name 'student_id'}) REFERENCES #{quote_table_name 'students'} (#{quote_column_name 'id'})"
   end
diff --git a/activerecord/test/schema/sqlite_specific_schema.rb b/activerecord/test/schema/sqlite_specific_schema.rb
index b7aff4f47d..b5552c2755 100644
--- a/activerecord/test/schema/sqlite_specific_schema.rb
+++ b/activerecord/test/schema/sqlite_specific_schema.rb
@@ -7,7 +7,7 @@ ActiveRecord::Schema.define do
   execute "DROP TABLE fk_test_has_pk" rescue nil
   execute <<_SQL
   CREATE TABLE 'fk_test_has_pk' (
-    'id' INTEGER NOT NULL PRIMARY KEY
+    'pk_id' INTEGER NOT NULL PRIMARY KEY
   );
 _SQL
 
@@ -16,7 +16,7 @@ _SQL
     'id'    INTEGER NOT NULL PRIMARY KEY,
     'fk_id' INTEGER NOT NULL,
 
-    FOREIGN KEY ('fk_id') REFERENCES 'fk_test_has_pk'('id')
+    FOREIGN KEY ('fk_id') REFERENCES 'fk_test_has_pk'('pk_id')
   );
 _SQL
-end
\ No newline at end of file
+end
diff --git a/activerecord/test/support/ddl_helper.rb b/activerecord/test/support/ddl_helper.rb
index 0107babaaf..43cb235e01 100644
--- a/activerecord/test/support/ddl_helper.rb
+++ b/activerecord/test/support/ddl_helper.rb
@@ -1,8 +1,8 @@
 module DdlHelper
   def with_example_table(connection, table_name, definition = nil)
-    connection.exec_query("CREATE TABLE #{table_name}(#{definition})")
+    connection.execute("CREATE TABLE #{table_name}(#{definition})")
     yield
   ensure
-    connection.exec_query("DROP TABLE #{table_name}")
+    connection.execute("DROP TABLE #{table_name}")
   end
 end
diff --git a/guides/source/4_2_release_notes.md b/guides/source/4_2_release_notes.md
index dd484ccee2..ef294f55d7 100644
--- a/guides/source/4_2_release_notes.md
+++ b/guides/source/4_2_release_notes.md
@@ -25,6 +25,31 @@ guide.
 Major Features
 --------------
 
+### Foreign key support
+
+The migration DSL now supports adding and removing foreign keys. They are dumped
+to `schema.rb` as well. At this time, only the `mysql`, `mysql2` and `postgresql`
+adapters support foreign keys.
+
+```ruby
+# add a foreign key to `articles.author_id` referencing `authors.id`
+add_foreign_key :articles, :authors
+
+# add a foreign key to `articles.author_id` referencing `users.lng_id`
+add_foreign_key :articles, :users, column: :author_id, primary_key: "lng_id"
+
+# remove the foreign key on `accounts.branch_id`
+remove_foreign_key :accounts, :branches
+
+# remove the foreign key on `accounts.owner_id`
+remove_foreign_key :accounts, column: :owner_id
+```
+
+See the API documentation on
+[add_foreign_key](http://api.rubyonrails.org/v4.2.0/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-add_foreign_key)
+and
+[remove_foreign_key](http://api.rubyonrails.org/v4.2.0/classes/ActiveRecord/ConnectionAdapters/SchemaStatements.html#method-i-remove_foreign_key)
+for a full description.
 
 
 Railties
diff --git a/guides/source/active_record_migrations.md b/guides/source/active_record_migrations.md
index fd2125424b..1b0ae3ec7e 100644
--- a/guides/source/active_record_migrations.md
+++ b/guides/source/active_record_migrations.md
@@ -452,6 +452,40 @@ Column modifiers can be applied when creating or changing a column:
 Some adapters may support additional options; see the adapter specific API docs
 for further information.
 
+### Foreign Keys
+
+While it's not required you might want to add foreign key constraints to
+[guarantee referential integrity](#active-record-and-referential-integrity).
+
+```ruby
+add_foreign_key :articles, :authors
+```
+
+This adds a new foreign key to the `author_id` column of the `articles`
+table. The key references the `id` column of the `articles` table. If the
+column names can not be derived from the table names, you can use the
+`:column` and `:primary_key` options.
+
+Rails will generate a name for every foreign key starting with
+`fk_rails_` followed by 10 random characters.
+There is a `:name` option to specify a different name if needed.
+
+NOTE: Active Record only supports single column foreign keys. `execute` and
+`structure.sql` are required to use composite foreign keys.
+
+Removing a foreign key is easy as well:
+
+```ruby
+# let Active Record figure out the column name
+remove_foreign_key :accounts, :branches
+
+# remove foreign key for a specific column
+remove_foreign_key :accounts, column: :owner_id
+
+# remove foreign key by name
+remove_foreign_key :accounts, name: :special_fk_name
+```
+
 ### When Helpers aren't Enough
 
 If the helpers provided by Active Record aren't enough you can use the `execute`
@@ -482,6 +516,7 @@ definitions:
 * `add_index`
 * `add_reference`
 * `add_timestamps`
+* `add_foreign_key`
 * `create_table`
 * `create_join_table`
 * `drop_table` (must supply a block)
@@ -507,24 +542,23 @@ migration what else to do when reverting it. For example:
 ```ruby
 class ExampleMigration < ActiveRecord::Migration
   def change
-    create_table :products do |t|
-      t.references :category
+    create_table :distributors do |t|
+      t.string :zipcode
     end
 
     reversible do |dir|
       dir.up do
-        #add a foreign key
+        # add a CHECK constraint
         execute <<-SQL
-          ALTER TABLE products
-            ADD CONSTRAINT fk_products_categories
-            FOREIGN KEY (category_id)
-            REFERENCES categories(id)
+          ALTER TABLE distributors
+            ADD CONSTRAINT zipchk
+              CHECK (char_length(zipcode) = 5) NO INHERIT;
         SQL
       end
       dir.down do
         execute <<-SQL
-          ALTER TABLE products
-            DROP FOREIGN KEY fk_products_categories
+          ALTER TABLE distributors
+            DROP CONSTRAINT zipchk
         SQL
       end
     end
@@ -538,7 +572,7 @@ end
 Using `reversible` will ensure that the instructions are executed in the
 right order too. If the previous example migration is reverted,
 the `down` block will be run after the `home_page_url` column is removed and
-right before the table `products` is dropped.
+right before the table `distributors` is dropped.
 
 Sometimes your migration will do something which is just plain irreversible; for
 example, it might destroy some data. In such cases, you can raise
@@ -561,16 +595,15 @@ made in the `up` method. The example in the `reversible` section is equivalent t
 ```ruby
 class ExampleMigration < ActiveRecord::Migration
   def up
-    create_table :products do |t|
-      t.references :category
+    create_table :distributors do |t|
+      t.string :zipcode
     end
 
-    # add a foreign key
+    # add a CHECK constraint
     execute <<-SQL
-      ALTER TABLE products
-        ADD CONSTRAINT fk_products_categories
-        FOREIGN KEY (category_id)
-        REFERENCES categories(id)
+      ALTER TABLE distributors
+        ADD CONSTRAINT zipchk
+        CHECK (char_length(zipcode) = 5);
     SQL
 
     add_column :users, :home_page_url, :string
@@ -582,11 +615,11 @@ class ExampleMigration < ActiveRecord::Migration
     remove_column :users, :home_page_url
 
     execute <<-SQL
-      ALTER TABLE products
-        DROP FOREIGN KEY fk_products_categories
+      ALTER TABLE distributors
+        DROP CONSTRAINT zipchk
     SQL
 
-    drop_table :products
+    drop_table :distributors
   end
 end
 ```
@@ -617,43 +650,27 @@ end
 The `revert` method also accepts a block of instructions to reverse.
 This could be useful to revert selected parts of previous migrations.
 For example, let's imagine that `ExampleMigration` is committed and it
-is later decided it would be best to serialize the product list instead.
-One could write:
+is later decided it would be best to use Active Record validations,
+in place of the `CHECK` constraint, to verify the zipcode.
 
 ```ruby
-class SerializeProductListMigration < ActiveRecord::Migration
+class DontUseConstraintForZipcodeValidationMigration < ActiveRecord::Migration
   def change
-    add_column :categories, :product_list
-
-    reversible do |dir|
-      dir.up do
-        # transfer data from Products to Category#product_list
-      end
-      dir.down do
-        # create Products from Category#product_list
-      end
-    end
-
     revert do
       # copy-pasted code from ExampleMigration
-      create_table :products do |t|
-        t.references :category
-      end
-
       reversible do |dir|
         dir.up do
-          #add a foreign key
+          # add a CHECK constraint
           execute <<-SQL
-            ALTER TABLE products
-              ADD CONSTRAINT fk_products_categories
-              FOREIGN KEY (category_id)
-              REFERENCES categories(id)
+            ALTER TABLE distributors
+              ADD CONSTRAINT zipchk
+                CHECK (char_length(zipcode) = 5);
           SQL
         end
         dir.down do
           execute <<-SQL
-            ALTER TABLE products
-              DROP FOREIGN KEY fk_products_categories
+            ALTER TABLE distributors
+              DROP CONSTRAINT zipchk
           SQL
         end
       end
@@ -918,10 +935,10 @@ that Active Record supports. This could be very useful if you were to
 distribute an application that is able to run against multiple databases.
 
 There is however a trade-off: `db/schema.rb` cannot express database specific
-items such as foreign key constraints, triggers, or stored procedures. While in
-a migration you can execute custom SQL statements, the schema dumper cannot
-reconstitute those statements from the database. If you are using features like
-this, then you should set the schema format to `:sql`.
+items such as triggers, or stored procedures. While in a migration you can
+execute custom SQL statements, the schema dumper cannot reconstitute those
+statements from the database. If you are using features like this, then you
+should set the schema format to `:sql`.
 
 Instead of using Active Record's schema dumper, the database's structure will
 be dumped using a tool specific to the database (via the `db:structure:dump`
@@ -948,7 +965,7 @@ Active Record and Referential Integrity
 ---------------------------------------
 
 The Active Record way claims that intelligence belongs in your models, not in
-the database. As such, features such as triggers or foreign key constraints,
+the database. As such, features such as triggers or constraints,
 which push some of that intelligence back into the database, are not heavily
 used.
 
@@ -957,14 +974,10 @@ which models can enforce data integrity. The `:dependent` option on
 associations allows models to automatically destroy child objects when the
 parent is destroyed. Like anything which operates at the application level,
 these cannot guarantee referential integrity and so some people augment them
-with foreign key constraints in the database.
-
-Although Active Record does not provide any tools for working directly with
-such features, the `execute` method can be used to execute arbitrary SQL. You
-can also use a gem like
-[foreigner](https://github.com/matthuhiggins/foreigner) which adds foreign key
-support to Active Record (including support for dumping foreign keys in
-`db/schema.rb`).
+with [foreign key constraints](#foreign-keys) in the database.
+
+Although Active Record does not provide all the tools for working directly with
+such features, the `execute` method can be used to execute arbitrary SQL.
 
 Migrations and Seed Data
 ------------------------