Patch MySQL to ignore PostgreSQL schema options

This ensures that options such as `using: :gin` and PostgreSQL operator
classes are ignored when loading a schema into a MySQL database.

config/initializers/mysql_ignore_postgresql_options.rb

+# This patches ActiveRecord so indexes created using the MySQL adapter ignore
+# any PostgreSQL specific options (e.g. `using: :gin`).
+#
+# These patches do the following for MySQL:
+#
+# 1. Indexes created using the :opclasses option are ignored (as they serve no
+#    purpose on MySQL).
+# 2. When creating an index with `using: :gin` the `using` option is discarded
+#    as :gin is not a valid value for MySQL.
+# 3. The `:opclasses` option is stripped from add_index_options in case it's
+#    used anywhere other than in the add_index methods.
+
+if defined?(ActiveRecord::ConnectionAdapters::Mysql2Adapter)
+  module ActiveRecord
+    module ConnectionAdapters
+      class Mysql2Adapter < AbstractMysqlAdapter
+        alias_method :__gitlab_add_index, :add_index
+        alias_method :__gitlab_add_index_sql, :add_index_sql
+        alias_method :__gitlab_add_index_options, :add_index_options
+
+        def add_index(table_name, column_name, options = {})
+          unless options[:opclasses]
+            __gitlab_add_index(table_name, column_name, options)
+          end
+        end
+
+        def add_index_sql(table_name, column_name, options = {})
+          unless options[:opclasses]
+            __gitlab_add_index_sql(table_name, column_name, options)
+          end
+        end
+
+        def add_index_options(table_name, column_name, options = {})
+          if options[:using] and options[:using] == :gin
+            options = options.dup
+            options.delete(:using)
+          end
+
+          if options[:opclasses]
+            options = options.dup
+            options.delete(:opclasses)
+          end
+
+          __gitlab_add_index_options(table_name, column_name, options)
+        end
+      end
+    end
+  end
+end