mongodb/mongoid · error · Mongoid::Errors::InvalidSetPolymorphicRelation

The attribute can't be set to an instance of as has…

Error message

The %{name} attribute can't be set to an instance of %{other_klass} as %{other_klass} has multiple associations referencing %{klass} as %{name}.

What it means

During bind_one of a polymorphic embedded_in, check_polymorphic_inverses! resolved more than one inverse on the parent class for the assigned document, so which parent association owns the child is ambiguous and the assignment is rejected.

Solutions

1. Set the relation through a specific parent association instead of the ambiguous setter
2. Define inverse_of or distinct as: names on the parent associations to disambiguate

When it happens

Trigger: Thrown at lib/mongoid/association/embedded/embedded_in/binding.rb:64 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of mongodb/mongoid@0da0e23d71 (2026-08-23). Data as JSON: /api/errors/b5bb12735bb4ad78. Report an issue: GitHub.

Appendix: source

Thrown at lib/mongoid/association/embedded/embedded_in/binding.rb:64

              end
            end
          end

          private

          # Check for problems with multiple inverse definitions.
          #
          # @api private
          #
          # @example Check for inverses errors.
          #   binding.check_inverses!(doc)
          #
          # @param [ Document ] doc The document to check.
          def check_polymorphic_inverses!(doc)
            return unless inverses = _association.inverses(doc)
            return unless inverses.length > 1

            raise Errors::InvalidSetPolymorphicRelation.new(
              _association.name, _base.class.name, _target.class.name
            )
          end
        end
      end
    end
  end
end

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