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Question

My model "Preconditions" creates relationships between records in the "Assignments" model, so that one assignment can be a pre-requisite for another assignment.

In the console, Assignment.find(3).preassigns outputs the second record from the Assignment table, as I expect. However, the inverse is not working. Assignment.find(2).mainassigns returns an empty set.

precondition.rb

class Precondition < ApplicationRecord
    belongs_to :mainassign, class_name: "Assignment"
    belongs_to :preassign, class_name: "Assignment"
end

assignment.rb

class Assignment < ApplicationRecord
    belongs_to  :seminar
    has_many    :scores, dependent: :destroy

    has_many    :preconditions, class_name: "Precondition",
                                foreign_key: "mainassign_id",
                                dependent: :destroy
    has_many    :mainconditions, class_name: "Precondition",
                                foreign_key: "preassign_id",
                                dependent: :destroy

    has_many    :preassigns, through: :preconditions, as: :mainassign, source: :preassign
    has_many    :mainassigns, through: :preconditions, as: :preassign, source: :mainassign

    validates :name, presence: true, length: { maximum: 40 }
    validates :seminar_id, presence: true
    validates :possible, presence: true
    validates_numericality_of   :possible, only_integer: true
end

Based on similar questions, I've tried adding a polymorphic tag in the Preconditions table, like so:

belongs_to :mainassign, class_name: "Assignment", polymorphic: true

This caused an error, but the log suggested that I try a source_type, so I tried adding that to the assignments model, like so:

has_many    :mainassigns, through: :preconditions, as: :preassign, source: :mainassign, source_type: "Assignment"

But that resulted in an SQL column not found error.

Thank you in advance for any insight.

Answer 1

No need for polymorphic here. You need to change :through value for mainassigns :

From:

has_many :mainassigns, through: :preconditions, as: :preassign, source: :mainassign

To:

has_many :mainassigns, through: :mainconditions, as: :preassign, source: :mainassign