connection.select_value 只返回带有 pg gem 的 postgres 中的字符串

Question

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?

Answer 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 .

Answer 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.