class ActiveRecord::AssociationTypeMismatch - activerecord-4.2.3 Documentation

# class ActiveRecord::AssociationTypeMismatch

Raised when an object assigned to an association has an incorrect type.

    class Ticket < ActiveRecord::Base
      has_many :patches
    end

    class Patch < ActiveRecord::Base
      belongs_to :ticket
    end

    # Comments are not patches, this assignment raises AssociationTypeMismatch.
    @ticket.patches << Comment.new(content: "Please attach tests to your patch.")