as_json works differently for preload and eagerload

[salmanasiddiqui]

Steps to reproduce

class Portfolio < ActiveRecord::Base
  has_many :tickets
end
class Ticket < ActiveRecord::Base
  belongs_to :portfolio
end

@tickets = Ticket.includes(:portfolio).select('tickets.*', 'SUBSTRING(tickets.title,1,4) as code')
@tickets.first.as_json
@tickets.references(:portfolio).first.as_json

Expected behavior

@tickets.first.as_json should be equal to @tickets.references(:portfolios).first.as_json

Actual behavior

@tickets.first.as_json includes the code (alias column defined in the select)
@tickets.references(:portfolios).first.as_json does not include the code (alias column defined in the select)

Note

The problem arises only when eager_load is called by includes, preload works fine. More to that, using joins works fine aswell.

System configuration

Rails version: 4.2.6

Ruby version: 2.1.8

[vipulnsward]

@salmanasiddiqui I did not get the use of references in this context? Its not being used.
The result of references vs includes is a bit different. In the former we need to construct results based on aliases, and don't happen to use select clauses, unlike includes, which is causing the difference.

[salmanasiddiqui]

I just wanted to show that eager_load and preload are producing different results. If we dont use references, instead use where(portfolios: {id: [3,2]}) the result will be same. As Rails will also use the eager_load instead of preload in that case.
are you saying that this is the correct behavior? Is selectignored incase when includes results in LEFT JOIN? Is this documented somewhere?

[vipulnsward]

Hi @salmanasiddiqui this isn't documented, but a known limitation. Going to mark this as a documentation issue.

[salmanasiddiqui]

Known limitation, u mean that this cant be fixed and there isnt any workaround aswell?

I don't think that documenting it, is enough. It should also log a warning that select is being ignored in such scenarios.

[sgrif]

This isn't even a limitation. This is the expected behavior. When you put an additional field in the select column, it is accessible through the record. I don't think this warrants a mention explicitly on as_json, and the behavior of select is thoroughly documented.

[vipulnsward]

@sgrif the issue is that select isn't returning extra selected values in case of eager load.

[sgrif]

Ah, I see now. It's a very confusingly set up report...

[salmanasiddiqui]

workaround for anyone else facing this issue

replace includes with preload and use joins

example:
@tickets = Ticket.preload(:portfolio).joins('LEFT JOIN portfolios on portfolios.id = tickets.portfolio_id').select('tickets.*', 'SUBSTRING(tickets.title,1,4) as code').order('portfolios.name')

[al2o3cr]

@sgrif Is this the same issue seen in #12423 and #15185?

[sgrif]

Looks like it, yes.