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.")