1 files changed, 11 insertions, 1 deletions
diff --git a/activerecord/lib/active_record/migration/compatibility.rb b/activerecord/lib/active_record/migration/compatibility.rb
index ae45ac7157..9d3652d63b 100644
--- a/activerecord/lib/active_record/migration/compatibility.rb
+++ b/activerecord/lib/active_record/migration/compatibility.rb
@@ -104,11 +104,21 @@ module ActiveRecord
 
       class V5_0 < V5_1
         def create_table(table_name, options = {})
-          if ActiveRecord::Base.connection.adapter_name == "PostgreSQL"
+          connection_name = self.connection.adapter_name
+          if connection_name == "PostgreSQL"
             if options[:id] == :uuid && !options[:default]
               options[:default] = "uuid_generate_v4()"
             end
           end
+
+          # Since 5.1 Postgres adapter uses bigserial type for primary
+          # keys by default and MySQL uses bigint. This compat layer makes old migrations utilize
+          # serial/int type instead -- the way it used to work before 5.1.
+          if options[:id].blank?
+            options[:id] = :integer
+            options[:auto_increment] = true
+          end
+
           super
         end
       end