Validate uniqueness of many to many association in Rails

Question

Say I have Project, that is in many-to-many association with Tag. I\'m using has_many through so I have separate join model.

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Answer 1

Try validates_associated.

That should, I believe, allow the join model validations to run before saving. So in your case:

class Project
   has many :tags, :through => :taggings
   validates_associated :taggings
end

class Taggings
   belongs_to :tags

   #your validations here....
end

class Tag
   has_many :taggings
end

Answer 2

I think what you want is validates_uniqueness_of:

class Taggings
  belongs_to :tags
  validates_uniqueness_of :tag_id, :scope => :project_id
end

This is what I'm using, and works well.