JOIN ON … AND conditions when eager loading association in ActiveRecord

Question

Please notice, this is not a question about adding a WHERE condition to an association but a rather advanced question about how to alter the JOIN clause when using eager

Answer 1

If I am right, you only want to eager load the subset of the associations. Well, in order to do so, you need to outsmart the rails . You can basically create another association such as "highest_grade" to fetch the grades with level A+ using a where clause inside the ruby lambda. This can be achieved by using the following snippet of code

class Parent
  has_many :children
  has_many :grades, through: :children
  has_many :highest_grade, ->{where(level: 'A+')}, class_name: "Grade", through: :children
end

Now you need to eager load the highest_grade association.

 Parent.eager_load(:highest_grades)

This will essentially load all the parents along with only those grades whose level is A+.

It generates the following query.

 SQL (0.3ms)  SELECT `parents`.`id` AS t0_r0, `parents`.`created_at` AS    t0_r1, `parents`.`updated_at` AS t0_r2, `grades`.`id` AS t1_r0,    `grades`.`child_id` AS t1_r1, `grades`.`created_at` AS t1_r2, `grades`.`updated_at` AS t1_r3, `grades`.`level` AS t1_r4 FROM `parents` LEFT OUTER JOIN `children` ON `children`.`parent_id` = `parents`.`id` LEFT OUTER JOIN `grades` ON `grades`.`child_id` = `children`.`id` AND `grades`.`level` = '

If you want to understand how does this work. Please use this link as a reference