Condition on Associations in rails

Unsaved objects and associations

You can manipulate objects and associations before they are saved to the database, but there is some special behavior you should be aware of, mostly involving the saving of associated objects.
You can set the :autosave option on a has_one, belongs_to, has_many, or has_and_belongs_to_many association. Setting it to true will always save the members, whereas setting it to false will never save the members. More details about :autosave option is available at AutosaveAssociation.

One-to-one associations

• Assigning an object to a has_one association automatically saves that object and the object being replaced (if there is one), in order to update their foreign keys - except if the parent object is unsaved (new_record? == true).
  
• If either of these saves fail (due to one of the objects being invalid), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.
  
• If you wish to assign an object to a has_one association without saving it, use the build_association method (documented below). The object being replaced will still be saved to update its foreign key.
  
• Assigning an object to a belongs_to association does not save the object, since the foreign key field belongs on the parent. It does not save the parent either.
  

Collections

• Adding an object to a collection (has_many or has_and_belongs_to_many) automatically saves that object, except if the parent object (the owner of the collection) is not yet stored in the database.
  
• If saving any of the objects being added to a collection (via push or similar) fails, then push returns false.
  
• If saving fails while replacing the collection (via association=), an ActiveRecord::RecordNotSaved exception is raised and the assignment is cancelled.
  
• You can add an object to a collection without automatically saving it by using the collection.build method (documented below).
  
• All unsaved (new_record? == true) members of the collection are automatically saved when the parent is saved.
  

 

Customizing the query

Associations are built from Relations, and you can use the Relation syntax to customize them. For example, to add a condition:

class Blog < ActiveRecord::Base
  has_many :published_posts, -> { where published: true }, class_name: 'Post'
end

 
Inside the -> { ... } block you can use all of the usual Relation methods.