Disallow chaining manual .selects + .pluck

[blnoonan]

Is your feature request related to a problem? Please describe.

In ActiveRecord, chaining a manual .select (.select called with raw SQL) and .pluck produces unexpected results. Examples of this:

• User.select('id AS id2').pluck(:id2) > Unknown column id2 (issue)
• User.select('id, email AS user_email').pluck('id', 'user_email') > Unknown column user_email
• Gift.select("DISTINCT(country_code)").pluck(:country_code) > Results are not de-duped (issue)

And other cases that work can be written more simply:

• User.select('id').pluck(:id) can be written as User.select(:id).pluck(:id)

The nature of select, which loads up an ActiveRecord::Relation, is incongruous with pluck, which operates on the DB directly. At best the combination can work incidentally, and at worst it can produce silently unexpected results. As stated in the linked rails issue:

pluck selects only what was passed as its arguments (and ignores manual selects)

Describe the solution you'd like

Add a rule to disallow chaining manual selects with pluck. Provide helpful messaging that pluck will ignore the manual directive provided to select.

I am happy to contribute this rule if folks agree it's valuable!

Describe alternatives you've considered

• Autocorrect .select("RAW").pluck to .select("RAW").to_a.pluck - technically works, but not always desired
• Disallow this behavior in rails (appears to be a no-go)
• Disallow any chaining of .select with .pluck - seems valuable, though perhaps overly broad