Rails - Using join with custom-named associations

问题

I have the following model

class Measurement < ApplicationRecord
  belongs_to :examination, class_name: "TestStructure", foreign_key: "examination_id"

end

The association is actually made to the TestStructure model, but the association name is examination. There is no examination table.

The problem arises when I'm querying using join. The following query

Measurement.joins(:examination).where(examination: { year: 2016, month: 5 })

fails, with this error

ActiveRecord::StatementInvalid:
   PG::UndefinedTable: ERROR:  missing FROM-clause entry for table "examination"
   LINE 1: ...d" = "measurements"."examination_id" WHERE "examinati...
# --- Caused by: ---
 # PG::UndefinedTable:
 #   ERROR:  missing FROM-clause entry for table "examination"
 #   LINE 1: ...d" = "measurements"."examination_id" WHERE "examinati...

So clearly, the examinations table doesn't exists, but I can't find a way to tell ActiveRecord I'm using a named association instead of the default one.

Any insights?

回答1:

where expects the actual table name, it just inserts it in SQL:

Article.where(whatever: {you: 'want'}).to_sql
=> "SELECT `articles`.* FROM `articles` WHERE `whatever`.`you` = 'want'"

So you may use:

Measurement.joins(:examination).where(test_structures: { year: 2016, month: 5 })

But it's not good

Then you depend on table name while Model should abstract such things. You could use merge:

Measurement.joins(:examination).merge(TestStructure.where(year: 2016, month: 5))

回答2:

For joins you use the association name, but for where you need to use the table name

Measurement.joins(:examination).where(test_structures: { year: 2016, month: 5 })

or

Measurement.joins(:examination).where('test_structures.year': 2016, 'test_structures.month': 5 )

回答3:

In this example table name examinations should be provided instead of an association name examination in the where method.

Measurement.jons(:examination).where(examinations: { year: 2016, month: 5 })

来源：https://stackoverflow.com/questions/45947285/rails-using-join-with-custom-named-associations