1 files changed, 75 insertions, 0 deletions
diff --git a/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb b/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb
new file mode 100644
index 0000000000..dd79bcf542
--- /dev/null
+++ b/activerecord/lib/rails/generators/active_record/migration/migration_generator.rb
@@ -0,0 +1,75 @@
+# frozen_string_literal: true
+
+require "rails/generators/active_record"
+
+module ActiveRecord
+  module Generators # :nodoc:
+    class MigrationGenerator < Base # :nodoc:
+      argument :attributes, type: :array, default: [], banner: "field[:type][:index] field[:type][:index]"
+
+      class_option :primary_key_type, type: :string, desc: "The type for primary key"
+      class_option :database, type: :string, aliases: %i(db), desc: "The database for your migration. By default, the current environment's primary database is used."
+
+      def create_migration_file
+        set_local_assigns!
+        validate_file_name!
+        migration_template @migration_template, File.join(db_migrate_path, "#{file_name}.rb")
+      end
+
+      private
+        attr_reader :migration_action, :join_tables
+
+        # Sets the default migration template that is being used for the generation of the migration.
+        # Depending on command line arguments, the migration template and the table name instance
+        # variables are set up.
+        def set_local_assigns!
+          @migration_template = "migration.rb"
+          case file_name
+          when /^(add)_.*_to_(.*)/, /^(remove)_.*?_from_(.*)/
+            @migration_action = $1
+            @table_name       = normalize_table_name($2)
+          when /join_table/
+            if attributes.length == 2
+              @migration_action = "join"
+              @join_tables      = pluralize_table_names? ? attributes.map(&:plural_name) : attributes.map(&:singular_name)
+
+              set_index_names
+            end
+          when /^create_(.+)/
+            @table_name = normalize_table_name($1)
+            @migration_template = "create_table_migration.rb"
+          end
+        end
+
+        def set_index_names
+          attributes.each_with_index do |attr, i|
+            attr.index_name = [attr, attributes[i - 1]].map { |a| index_name_for(a) }
+          end
+        end
+
+        def index_name_for(attribute)
+          if attribute.foreign_key?
+            attribute.name
+          else
+            attribute.name.singularize.foreign_key
+          end.to_sym
+        end
+
+        def attributes_with_index
+          attributes.select { |a| !a.reference? && a.has_index? }
+        end
+
+        # A migration file name can only contain underscores (_), lowercase characters,
+        # and numbers 0-9. Any other file name will raise an IllegalMigrationNameError.
+        def validate_file_name!
+          unless /^[_a-z0-9]+$/.match?(file_name)
+            raise IllegalMigrationNameError.new(file_name)
+          end
+        end
+
+        def normalize_table_name(_table_name)
+          pluralize_table_names? ? _table_name.pluralize : _table_name.singularize
+        end
+    end
+  end
+end