Fix eager loading association using default_scope for finder methods.

[mechanicles]

• Eager loading was not working for the default_scope (class method)
  for 'find' & 'find_by' methods.
• Fixed these by adding a new check 'respond_to?(:default_scope)'.

[mechanicles]

@rafaelfranca Sorry I don't know to whom to address this PR currently. So I pinged you.

Actually yesterday I sent this PR. But today morning, I saw that Travis CI got failed. I checked that and my PR got failed mostly for jruby and rbx related builds. Today I rebased with master and did force push to my branch. But now Travis doesn't show any status. Let me know if I missed something.

[rafaelfranca]

No problem, Is this PR fixing any issue?

[mechanicles]

@rafaelfranca I just checked that is there any similar issues to this issue. But didn't get any. Actually I found this issue while testing this file activerecord/test/cases/associations/eager_test.rb and found that eager loading is not working for some finder methods if we use default_scope as class method.

[rafaelfranca]

Should not we only return if it is a eager loading?

[mechanicles]

@rafaelfranca I'm not getting your point. Could you elaborate more on it?.

Also one more thing, I have added respond_to?(:default_scope) in two places in this PR. We already have a method scope_attributes? in scoping/default.rb. I think, it's good place to move respond_to?(:default_scope) over there.

something like this,

 # Are there attributes associated with this scope?
def scope_attributes? # :nodoc:
  super || default_scopes.any? || respond_to?(:default_scope)
end

Let me know your thoughts

[mechanicles]

@rafaelfranca Also it is not just about eager loading. Actually problem is, if we define default_scope as class method, then it doesn't work for finder methods find and find_by.

class Post
  def self.default_scope
    where(published: true)
  end
end

Suppose posts table has columns like category and published.

If we try to search a post like,

Post.find_by(category: 'rails')

Then here 'default_scope' doesn't work. It doesn't not load published: true in the query.

[rafaelfranca]

Right, now I get the problem.

Yeah, I think we should move the respond_to? to that method.

[mechanicles]

@rafaelfranca I have update code and moved respond_to?(:default_scope) to scope_attributes? method.