问题
I'm converting a rails app from using mysql (mysql2 gem) to postgres (pg gem).
With mysql, ActiveRecord::Base.connection.select_value calls return values typed according to the data, for example:
> ActiveRecord::Base.connection.select_value("SELECT COUNT(*) FROM errors")
=> 86
> ActiveRecord::Base.connection.select_value("SELECT exception FROM errors where id=565")
=> "TechTalk.Genome.SqlExecutionException"
> ActiveRecord::Base.connection.select_value("SELECT id FROM errors where id=565")
=> 565
However, with postgres, connection.select_value always returns a string:
> ActiveRecord::Base.connection.select_value("SELECT COUNT(*) FROM errors")
=> "1"
> ActiveRecord::Base.connection.select_value("SELECT id FROM errors")
=> "1"
> ActiveRecord::Base.connection.select_value("SELECT source FROM errors limit 1")
=> "webapp"
This broke a few unit tests, and while those are fixable, I'm certain we have other code relying on these return values. Is there a way to get properly-typed return values from connection.select_value when using postgres?
回答1:
The short answer is no. The 'pg' driver intentionally provides as thin a layer as possible on top of the native 'libpq' driver. It doesn't do typecasting, as that's the responsibility of higher-level libraries that have some insight into the domain in which the results will be used. The rationale for this decision is documented on the PostgreSQL Wiki, and I'd be happy to discuss it with you further on the mailing list.
回答2:
For those who land here looking for an activerecord attributes (Rails) specific answer: I've done some tests (https://gist.github.com/gamov/8fe38733012931eb3360) and discovered that:
RequestedItem.where(id: 1).select(*, 10 AS tq).first.tq.class
returns a String with Rails < 4 and a Fixnum with Rails >= 4.
The Postgres adapter will forward the types from the DB to the Persistence module so casting can be done transparently.
来源：https://stackoverflow.com/questions/12571215/connection-select-value-only-returns-strings-in-postgres-with-pg-gem