Adding Add method to ActiveRecord array

Question

I have the models Category and Products. If I use category.products << new_product the item gets added to the array and the record is saved to the database. I tried adding the following "add" method to the array class and while it does adds the new_product to the array, it does not save it to the database. Why is that?

class Array
  def add(item)
    self << item
  end
end

Update:

collection_proxy.rb has the following method:

def <<(*records)
  proxy_association.concat(records) && self
end
alias_method :push, :<<

So the following extension works:

class ActiveRecord::Relation
  def add(*records)
    proxy_association.concat(records) && self
  end
end

Solution:

Add an alias to the CollectionProxy:

class ActiveRecord::Associations::CollectionProxy
  alias_method :add, :<<
end

Because Rails associations aren't arrays, they just claim they are. — Dave Newton
– Dave Newton, Commented Aug 15, 2012 at 21:37

Kyle · Accepted Answer · 2012-08-16 16:34:11Z

2

Edit: Manuel found a better solution

class ActiveRecord::Associations::CollectionProxy
  alias_method :add, :<<
end

Original solution:

This should get you started. It's not perfect.

class ActiveRecord::Relation
  def add(attrs)
    create attrs
  end
end

Rather than fire up a new rails project with your model names, I just used one I had for the following example:

1.9.3p194 :006 > Artist.create(:first_name => "Kyle", :last_name => "G", :email => "[email protected]")
 => #<Artist id: 5, first_name: "Kyle", last_name: "G", nickname: nil, email: "[email protected]", created_at: "2012-08-16 04:08:30", updated_at: "2012-08-16 04:08:30", profile_image_id: nil, active: true, bio: nil> 
1.9.3p194 :007 > Artist.first.posts.count
 => 0 
1.9.3p194 :008 > Artist.first.posts.add :title => "Foo", :body => "Bar"
 => #<Post id: 12, title: "Foo", body: "Bar", artist_id: 5, created_at: "2012-08-16 04:08:48", updated_at: "2012-08-16 04:08:48"> 
1.9.3p194 :009 > Artist.first.posts.count
 => 1

edited Aug 16, 2012 at 16:34

answered Aug 16, 2012 at 4:11

Kyle

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8 Comments

Manuel Over a year ago

Thanks. Do you know why it fails when you add the new record not as a hash but passing an object? (I'm getting "NoMethodError: undefined method `stringify_keys'")

Kyle Over a year ago

@Manuel yes, add is calling create and it expects attributes as a hash. You could alter the method to something like create item.attributes but you may get some problems with protected attributes. Consider picking out the ones you need from the object.

Manuel Over a year ago

Not quite :( create item.attributes creates the record in the database but does not make the relation (in your case artist_id is null).

Kyle Over a year ago

@Manuel Remove the relation id from the hash first. item.attributes.delete(:category_id). It was set to nil and the create used that as the value. If it was missing from the hash, the relation would be setup.

Manuel Over a year ago

We're almost there. Please see update. Do you know how to call the << method of the collection instead of copy/pasting its contents? I've tried many things to no avail.

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