has_many through while selecting extra columns breaks in 4.1

[jshirley]

I have a relationship that includes selected columns (I'm honestly doubting whether this is a good idea at all now that I've ran into this), and it worked through 4.0.x. Updating the app to 4.1.0rc1 showed this behavior no longer works and results in an SQL error, as the select is not dropped from the query constructed.

Essentially, creating a relationship in this fashion (my actual use case is more sane, at least in theory, separating two very distinct classifications of relationships but also wanting to get the join table properties as well):

  has_many :prime_categories,
    -> {
      includes(:post_category)
      .references(:post_category)
      .where(post_categories: { prime: true })
      .select("post_categories.id, categories.*")
    },
    through: :post_categories,
    source: :category

In the gist I also have commented out the explicit includes and references, as they don't make a difference in 4.1.

Failing executable gist:
https://gist.github.com/jshirley/9744173

Note: This example does pass with activerecord 4.0.0 activated (the select is not included in the query).

[rafaelfranca]

Yes, This behavior changed and it is documented in the CHANGELOG. The reason is sometime someone want to select being applied (and it make sense) and how it worked before doesn't allow this. Now select are applied unless you choose to not.

[jshirley]

Can you confirm that the correct fix in the app to include .count(:all)?

This wasn't adequately explained in the CHANGELOG (in fact, searching for select in the 4.1 CHANGELOG doesn't yield any results at all). (Though it seems this is the right PR: #10710)

[rafaelfranca]

@jshirley yes, it is exactly this change. The CHANGELOG is there and it mentions select