multiple select only keep the last one

Combine those two selects into one: Event.select('title, location_title').limit(1)

this just a demo.
I won't use Event.select('title').select('location_title') directly. I just demonstrate the situation.

I have two named scope

I don't wanna transfer all attributes through the mobile network, so I create a 'simple' scope

scope :tiny, select('title, location')

if the client parameters include 'latitude' and 'longitude', then I'd like get a distance

scope :near, lambda { |lat, lon| select("((latitude-#{lat})(latitude-#{lat}) + (longitude-#{lon})(longitude-#{lon})) AS distance").order('distance DESC') }

now, I wanna tiny and near(30, 120)

irb(main):008:0> e = Event.tiny.near(30,120).first
=> #<Event >
irb(main):009:0> e.distance
=> 1660.7118672780507
irb(main):010:0> e.title
ActiveModel::MissingAttributeError: missing attribute: title

Shouldn't this

select("((latitude-#{lat})(latitude-#{lat}) + (longitude-#{lon})(longitude-#{lon})) AS distance")

be

select("(title, location, (latitude-#{lat})(latitude-#{lat}) + (longitude-#{lon})(longitude-#{lon})) AS distance")

Because as the example I gave above, select overrides previous select. Can you try the above and see if it works?

Also are you using some gem/plugin?

yes, this work

select("(title, location, (latitude-#{lat})(latitude-#{lat}) + (longitude-#{lon})(longitude-#{lon})) AS distance")

but if I have many scope, very_tiny, tiny, fast, slow

then I wanna 'very_tiny and near', 'tiny and near', 'fast and tiny and near'...

I must duplicate so many times in select, why I can't combine select just like combine where?

Attached is failing test.

of course that will do and it is much concise.

Neeraj do you want to upload the patch? is yours ;)

Ignore me if I'm wrong but isn't this an Arel issue?

In arel this is what you need to do

Table(:users).predicate(users[:id],users[:name])

AR needs to be fixed to pass columns in the right way.

I am working on a fix. The fix so far I have is

last = selects.last
if last.is_a?(Arel::Count) || (last.is_a?(String) && last.downcase[0,5] =~ /count/)
  arel = arel.project(selects.last)
else          
  arel = arel.project(*selects)
end

Above fix is passing all the tests (old and new) except one.

 Developer.send(:with_scope, :find => { :select => "id, name" }) do
    developer = Developer.find(:first, :select => 'id, salary', :conditions => "name = 'David'")
    assert_equal 80000, developer.salary
    assert !developer.has_attribute?(:name)
  end
end

I have a feeling that fixing the above test would require a lot of effort.

Will update on my findings soon.

Nice work Neeraj :)

Attached is a patch which fixes the issue and the test provided by Santiago(with order column added for postgresql).

However the patch is failing following test.

  def test_options_select_replaces_scope_select
    Developer.send(:with_scope, :find => { :select => "id, name" }) do
      developer = Developer.find(:first, :select => 'id, salary', :conditions => "name = 'David'")
      assert_equal 80000, developer.salary
      assert !developer.has_attribute?(:name)
    end
  end

This brings to question that I would like to discuss with larger community. Take a look at following two cases.

Post.select('title').select('author_name').first

Post.send(:with_scope, :find => {:select => 'title'}) do
  Post.find(:first, :select => 'author_name')
end

In the first case we would like to have both the selects. In the second case we would like to have only the second select.

When the execution for second select starts then all there is to act upon is a relation object which is already built using first select. Now there is no way to know if the second select should override the first select or should it be appeneded.

Current code only picks up the last select and hence the above mentioned test passes. However my patch picks up all the selects and test fails.

In order to fix this issue ,it seems, relation object needs to be aware of in what context a select was built into relation object.

In my first example relation object is being built using standard way and in that case second select should add to the first select.

In the second example relation object is being built using 'with_scope' and in that case second select should override first select. Howver in order to do that relation object needs to be aware that select was built using 'with_scope'. And given the level of indirection that is no easy task.

Would like to know thoughts of others on this issue. I just started looking whole AR, AM and Arel . So others are likely to have more insight into this issue.

Neeraj,

I discussed this issue with JK and we agreed on the following methods:

• select - should always concatenate
• order - should always concatenate
• reorder - overwrite order

We decided not to provide something as "reselect" yet, because we would like to wait a real use case coming from it. That said, it's ok to break the case in your example above. And we can provide a solution in the future if required.

Just one note, Pastorino is also working on a solution so I would love if you guys could sync. I'm also available if you guys need help.

Neeraj i was digging on this bug and the issue is on ARel.
I will fix this thing on ARel and then we need to apply only this patch on Rails.

I've done the new reorder method and i'm working on the order and select concatenation that's an ARel issue so perhaps Neeraj we can chat and see if you have a nice advance on the issue we can merge and if not i don't know :P. Perhaps try to solve another one or keep digging on this :D.

Santiago: Can you briefly outline what kind of change is forthcoming in Arel.

It seems that your arel patch will support both select cases I outlined in my previous comment.

When a select is invoked on an existing relation object then how are you going to decide whether to append or to override given that a relation object does not carry the context in which it applied 'select'.

A new and simpler API of ARel will be really nice. In this way people not using rails will be able to take advantage of good stuff ARel has to offer.

However I have a different question.

José Valim suggested that we can drop the offending test. You are suggesting that with your Arel fix that test does not need to be dropped.

# case 1
Post.select('title').select('author_name').first

# case 2
Post.send(:with_scope, :find => {:select => 'title'}) do
  Post.find(:first, :select => 'author_name')
end

In case 1 we need concatenation. In case 2 the second select should override.

What I would like to know is how would you find out that in case2 the relation object is built within 'with_scope' given that relation object does not carry the context in which it was create.

Yes we should do further talking offline :-)

Neeraj in both cases we need to concatenate

I've created another ticket to push the order related things here https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets...
I'm about to finish it.