Extract #build_create_index_definition

Exposes CreateIndex schema definition through new API, and stores ddl
on the schema definition. Refactors existing #add_index methods to use
this as an intermediate method to build the create index definition.

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

@@ -106,7 +106,8 @@ module ActiveRecord
           sql << "(#{quoted_columns(index)})"
           sql << "WHERE #{index.where}" if supports_partial_index? && index.where
 
-          sql.join(" ")
+          sql = sql.join(" ")
+          o.ddl = sql
         end
 
         def visit_CheckConstraintDefinition(o)

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

@@ -94,7 +94,7 @@ module ActiveRecord
 
     ChangeColumnDefinition = Struct.new(:column, :name) # :nodoc:
 
-    CreateIndexDefinition = Struct.new(:index, :algorithm, :if_not_exists) # :nodoc:
+    CreateIndexDefinition = Struct.new(:index, :algorithm, :if_not_exists, :ddl) # :nodoc:
 
     PrimaryKeyDefinition = Struct.new(:name) # :nodoc:
 

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

@@ -834,10 +834,21 @@ module ActiveRecord
       #
       # For more information see the {"Transactional Migrations" section}[rdoc-ref:Migration].
       def add_index(table_name, column_name, **options)
+        create_index = build_create_index_definition(table_name, column_name, **options)
+        execute(create_index.ddl)
+      end
+
+      # Builds a CreateIndexDefinition object.
+      #
+      # This definition object contains information about the index that would be created
+      # if the same arguments were passed to #add_index. See #add_index for information about
+      # passing a +table_name+, +column_name+, and other additional options that can be passed.
+      def build_create_index_definition(table_name, column_name, **options) # :nodoc:
         index, algorithm, if_not_exists = add_index_options(table_name, column_name, **options)
 
         create_index = CreateIndexDefinition.new(index, algorithm, if_not_exists)
-        execute schema_creation.accept(create_index)
+        schema_creation.accept(create_index)
+        create_index
       end
 
       # Removes the given index from the table.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -377,12 +377,20 @@ module ActiveRecord
       end
 
       def add_index(table_name, column_name, **options) # :nodoc:
+        create_index = build_create_index_definition(table_name, column_name, **options)
+        return unless create_index
+
+        execute(create_index.ddl)
+      end
+
+      def build_create_index_definition(table_name, column_name, **options) # :nodoc:
         index, algorithm, if_not_exists = add_index_options(table_name, column_name, **options)
 
         return if if_not_exists && index_exists?(table_name, column_name, name: index.name)
 
         create_index = CreateIndexDefinition.new(index, algorithm)
-        execute schema_creation.accept(create_index)
+        schema_creation.accept(create_index)
+        create_index
       end
 
       def add_sql_comment!(sql, comment) # :nodoc:

activerecord/lib/active_record/connection_adapters/mysql/schema_creation.rb

@@ -27,7 +27,7 @@ module ActiveRecord
           def visit_CreateIndexDefinition(o)
             sql = visit_IndexDefinition(o.index, true)
             sql << " #{o.algorithm}" if o.algorithm
-            sql
+            o.ddl = sql
           end
 
           def visit_IndexDefinition(o, create = false)

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

@@ -447,13 +447,20 @@ module ActiveRecord
         end
 
         def add_index(table_name, column_name, **options) # :nodoc:
+          create_index = build_create_index_definition(table_name, column_name, **options)
+          result = execute(create_index.ddl)
+
+          index = create_index.index
+          execute "COMMENT ON INDEX #{quote_column_name(index.name)} IS #{quote(index.comment)}" if index.comment
+          result
+        end
+
+        def build_create_index_definition(table_name, column_name, **options) # :nodoc:
           index, algorithm, if_not_exists = add_index_options(table_name, column_name, **options)
 
           create_index = CreateIndexDefinition.new(index, algorithm, if_not_exists)
-          result = execute schema_creation.accept(create_index)
-
-          execute "COMMENT ON INDEX #{quote_column_name(index.name)} IS #{quote(index.comment)}" if index.comment
-          result
+          schema_creation.accept(create_index)
+          create_index
         end
 
         def remove_index(table_name, column_name = nil, **options) # :nodoc:

activerecord/test/cases/migration/schema_definitions_test.rb

@@ -35,6 +35,32 @@ module ActiveRecord
         assert id_column.type
         assert id_column.sql_type
       end
+
+      def test_build_create_index_definition
+        connection.create_table(:test) do |t|
+          t.column :foo, :string
+        end
+        create_index = connection.build_create_index_definition(:test, :foo)
+
+        assert_match "CREATE INDEX", create_index.ddl
+        assert_equal "index_test_on_foo", create_index.index.name
+      ensure
+        connection.drop_table(:test) if connection.table_exists?(:test)
+      end
+
+      if current_adapter?(:Mysql2Adapter)
+        def test_build_create_index_definition_for_existing_index
+          connection.create_table(:test) do |t|
+            t.column :foo, :string
+          end
+          connection.add_index(:test, :foo)
+
+          create_index = connection.build_create_index_definition(:test, :foo, if_not_exists: true)
+          assert_nil create_index
+        ensure
+          connection.drop_table(:test) if connection.table_exists?(:test)
+        end
+      end
     end
   end
 end