2.3.1 :001 > class SchemaMigration < ActiveRecord::Base; self.primary_key = :version; end
 => :version 
class SchemaMigration < ActiveRecord::Base; self.primary_key = :version; end


2.3.1 :002 > SchemaMigration.first
  SchemaMigration Load (2.1ms)  SELECT  "schema_migrations".* FROM "schema_migrations"  ORDER BY "schema_migrations"."version" ASC LIMIT 1
 => #<SchemaMigration version: "20160727113335">