using #joins does not imply readonly = true.

[senny]

Fixes #10615

Before this patch using #joins resulted in a readonly result. However the executed query only selects the columns from the primary table. For example:

Author.joins(:posts) # SELECT authors.* FROM ...

As far as I can tell there is no reason we need to mark these records as readonly.

[senny]

@rafaelfranca @jonleighton can you take a look?

[senny]

@neerajdotname maybe you could also take a look?

[neerajsingh0101]

I personally find that implicit_readonly to be a bit broken in the current state. For example in the following case save! should have blown up. But it does not in master.

class User < ActiveRecord::Base

  has_many :todos

  def self.lab
    User.delete_all
    Todo.delete_all
    user = User.create!
    user.todos.create(title: 'clean house')

    user = User.joins(:todos).select("users.*, todos.title as todos_title").first

    puts user.readonly? #> false
    user.todos_title = 'clean pet'
    user.save! # no error but the title is not changed in the db
  end
end

Finding out all the cases were implicit_readonly should be applied is a bit tricky. So it is best if it is left to the user.

@senny while you are at it, please also cleanup usage of implicit_readonly also from relation.rb file.

Add something like this to change log

Usage of `implicit_readonly` is being removed`. Please use `readonly` method 
explicitly to mark records as `readonly.

Example:
user = User.joins(:todos).select("users.*, todos.title as todos_title").readonly(true).first
user.todos_title = 'clean pet'
user.save! # will raise error

I'm +1 on this PR.

[rafaelfranca]

@senny is it marked as readonly with explicit select for a join table field?

[neerajsingh0101]

@rafaelfranca if there is any select then @implicit_readonly = false.

https://github.com/rails/rails/blob/master/activerecord/lib/active_record/relation/query_methods.rb#L964

Parsing the select statement to see which of the columns belong to primary table and which ones belong to join table will be brittle and error-prone. Hence my take that lets leave it upto the user.

[pixeltrix]

Parsing the select statement to see which of the columns belong to primary table and which ones belong to join table will be brittle and error-prone. Hence my take that lets leave it upto the user.

👍 to this as it's the approach taken with the references method added to the Active Record Query API.

[senny]

@neerajdotname I removed the references from relation.rb can you take another look?

[rafaelfranca]

Seems good. We need to add this to the upgrading guide too. (Better to start soon)

[neerajsingh0101]

@senny looks good. +1 from me.

[senny]

@rafaelfranca what do you want to have covered in the guides? you no longer need readonly when joins was called. You should apply it when you use a custom select but you would have to do that until now. As I see it there is not really a change you need to upgrade to.