Add migration versioning via Migration subclasses

5 files changed, 64 insertions, 2 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index 8cedfd5277..87420a0746 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,9 @@
+*   Version the API presented to migration classes, so we can change parameter
+    defaults without breaking existing migrations, or forcing them to be
+    rewritten through a deprecation cycle.
+
+    *Matthew Draper*, *Ravil Bayramgalin*
+
 *   Use bind params for `limit` and `offset`. This will generate significantly
     fewer prepared statements for common tasks like pagination. To support this
     change, passing a string containing a comma to `limit` has been deprecated,
diff --git a/activerecord/lib/active_record/migration.rb b/activerecord/lib/active_record/migration.rb
index ca2537cdc3..53a84c1342 100644
--- a/activerecord/lib/active_record/migration.rb
+++ b/activerecord/lib/active_record/migration.rb
@@ -476,6 +476,32 @@ module ActiveRecord
   # are in a Migration with <tt>self.disable_ddl_transaction!</tt>.
   class Migration
     autoload :CommandRecorder, 'active_record/migration/command_recorder'
+    autoload :Compatibility, 'active_record/migration/compatibility'
+
+    # This must be defined before the inherited hook, below
+    class Current < Migration # :nodoc:
+    end
+
+    def self.inherited(subclass) # :nodoc:
+      super
+      if subclass.superclass == Migration
+        subclass.include Compatibility::Legacy
+      end
+    end
+
+    def self.[](version)
+      version = version.to_s
+      name = "V#{version.tr('.', '_')}"
+      unless Compatibility.const_defined?(name)
+        versions = Compatibility.constants.grep(/\AV[0-9_]+\z/).map { |s| s.to_s.delete('V').tr('_', '.').inspect }
+        raise "Unknown migration version #{version.inspect}; expected one of #{versions.sort.join(', ')}"
+      end
+      Compatibility.const_get(name)
+    end
+
+    def self.current_version
+      Rails.version.to_f
+    end
 
     MigrationFilenameRegexp = /\A([0-9]+)_([_a-z0-9]*)\.?([_a-z0-9]*)?\.rb\z/ # :nodoc:
 
diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
new file mode 100644
index 0000000000..71d702e812
--- /dev/null
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -0,0 +1,30 @@
+module ActiveRecord
+  class Migration
+    module Compatibility # :nodoc: all
+      V5_0 = Current
+
+      module FourTwoShared
+      end
+
+      class V4_2 < V5_0
+        # 4.2 is defined as a module because it needs to be shared with
+        # Legacy. When the time comes, V5_0 should be defined straight
+        # in its class.
+        include FourTwoShared
+      end
+
+      module Legacy
+        include FourTwoShared
+
+        def run(*)
+          ActiveSupport::Deprecation.warn \
+            "Directly inheriting from ActiveRecord::Migration is deprecated. " \
+            "Please specify the Rails release the migration was written for:\n" \
+            "\n" \
+            "  class #{self.class.name} < ActiveRecord::Migration[4.2]"
+          super
+        end
+      end
+    end
+  end
+end
diff --git a/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb b/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb
index fadab2a1e6..5f7201cfe1 100644
--- a/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/templates/create_table_migration.rb
@@ -1,4 +1,4 @@
-class <%= migration_class_name %> < ActiveRecord::Migration
+class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
   def change
     create_table :<%= table_name %><%= primary_key_type %> do |t|
 <% attributes.each do |attribute| -%>
diff --git a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
index 23a377db6a..107f107dc4 100644
--- a/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
+++ b/activerecord/lib/rails/generators/active_record/migration/templates/migration.rb
@@ -1,4 +1,4 @@
-class <%= migration_class_name %> < ActiveRecord::Migration
+class <%= migration_class_name %> < ActiveRecord::Migration[<%= ActiveRecord::Migration.current_version %>]
 <%- if migration_action == 'add' -%>
   def change
 <% attributes.each do |attribute| -%>