Add a `foreign_key` option to `references` while creating the table

Rather than having to do:

    create_table :posts do |t|
      t.references :user
    end

    add_foreign_key :posts, :users

You can instead do:

    create_table :posts do |t|
      t.references :user, foreign_key: true
    end

Similar to the `index` option, you can also pass a hash. This will be
passed as the options to `add_foreign_key`. e.g.:

    create_table :posts do |t|
      t.references :user, foreign_key: { primary_key: :other_id }
    end

is equivalent to

    create_table :posts do |t|
      t.references :user
    end

    add_foreign_key :posts, :users, primary_key: :other_id

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -94,11 +94,12 @@ class TableDefinition
       # An array of ColumnDefinition objects, representing the column changes
       # that have been defined.
       attr_accessor :indexes
-      attr_reader :name, :temporary, :options, :as
+      attr_reader :name, :temporary, :options, :as, :foreign_keys
 
       def initialize(types, name, temporary, options, as = nil)
         @columns_hash = {}
         @indexes = {}
+        @foreign_keys = {}
         @native = types
         @temporary = temporary
         @options = options
@@ -286,6 +287,10 @@ def index(column_name, options = {})
         indexes[column_name] = options
       end
 
+      def foreign_key(table_name, options = {}) # :nodoc:
+        foreign_keys[table_name] = options
+      end
+
       # Appends <tt>:datetime</tt> columns <tt>:created_at</tt> and
       # <tt>:updated_at</tt> to the table. See SchemaStatements#add_timestamps
       #
@@ -297,9 +302,12 @@ def timestamps(*args)
         column(:updated_at, :datetime, options)
       end
 
-      # Adds a reference. Optionally adds a +type+ column, if <tt>:polymorphic</tt> option is provided.
-      # <tt>references</tt> and <tt>belongs_to</tt> are acceptable. The reference column will be an +integer+
-      # by default, the <tt>:type</tt> option can be used to specify a different type.
+      # Adds a reference. Optionally adds a +type+ column, if
+      # <tt>:polymorphic</tt> option is provided.  <tt>references</tt> and
+      # <tt>belongs_to</tt> are acceptable. The reference column will be an
+      # +integer+ by default, the <tt>:type</tt> option can be used to specify
+      # a different type. A foreign key will be created if a +foreign_key+
+      # option is passed.
       #
       #  t.references(:user)
       #  t.references(:user, type: "string")
@@ -310,11 +318,18 @@ def references(
         *args,
         polymorphic: false,
         index: false,
+        foreign_key: false,
         type: :integer,
         **options
       )
         polymorphic_options = polymorphic.is_a?(Hash) ? polymorphic : options
         index_options = index.is_a?(Hash) ? index : {}
+        foreign_key_options = foreign_key.is_a?(Hash) ? foreign_key : {}
+
+        if polymorphic && foreign_key
+          raise ArgumentError, "Cannot add a foreign key on a polymorphic relation"
+        end
+
         args.each do |col|
           column("#{col}_id", type, options)
 
@@ -325,6 +340,10 @@ def references(
           if index
             self.index(polymorphic ? %w(type id).map { |t| "#{col}_#{t}" } : "#{col}_id", index_options)
           end
+
+          if foreign_key
+            self.foreign_key(col.to_s.pluralize, foreign_key_options)
+          end
         end
       end
       alias :belongs_to :references

activerecord/lib/active_record/connection_adapters/abstract/schema_statements.rb

@@ -204,7 +204,17 @@ def create_table(table_name, options = {})
         end
 
         result = execute schema_creation.accept td
-        td.indexes.each_pair { |c, o| add_index(table_name, c, o) } unless supports_indexes_in_create?
+
+        unless supports_indexes_in_create?
+          td.indexes.each_pair do |column_name, index_options|
+            add_index(table_name, column_name, index_options)
+          end
+        end
+
+        td.foreign_keys.each_pair do |other_table_name, foreign_key_options|
+          add_foreign_key(table_name, other_table_name, foreign_key_options)
+        end
+
         result
       end
 

activerecord/test/cases/migration/references_foreign_key_test.rb

@@ -0,0 +1,60 @@
+require 'cases/helper'
+
+if ActiveRecord::Base.connection.supports_foreign_keys?
+module ActiveRecord
+  class Migration
+    class ReferencesForeignKeyTest < ActiveRecord::TestCase
+      setup do
+        @connection = ActiveRecord::Base.connection
+        @connection.transaction do
+          @connection.create_table(:testing_parents, force: true)
+        end
+      end
+
+      teardown do
+        @connection.execute("drop table if exists testings")
+        @connection.execute("drop table if exists testing_parents")
+      end
+
+      test "foreign keys can be created with the table" do
+        @connection.create_table :testings do |t|
+          t.references :testing_parent, foreign_key: true
+        end
+
+        fk = @connection.foreign_keys("testings").first
+        assert_equal "testings", fk.from_table
+        assert_equal "testing_parents", fk.to_table
+      end
+
+      test "no foreign key is created by default" do
+        @connection.create_table :testings do |t|
+          t.references :testing_parent
+        end
+
+        assert_equal [], @connection.foreign_keys("testings")
+      end
+
+      test "options hash can be passed" do
+        @connection.change_table :testing_parents do |t|
+          t.integer :other_id
+          t.index :other_id, unique: true
+        end
+        @connection.create_table :testings do |t|
+          t.references :testing_parent, foreign_key: { primary_key: :other_id }
+        end
+
+        fk = @connection.foreign_keys("testings").find { |k| k.to_table == "testing_parents" }
+        assert_equal "other_id", fk.primary_key
+      end
+
+      test "foreign keys cannot be added to polymorphic relations when creating the table" do
+        @connection.create_table :testings do |t|
+          assert_raises(ArgumentError) do
+            t.references :testing_parent, polymorphic: true, foreign_key: true
+          end
+        end
+      end
+    end
+  end
+end
+end