Linking tables across multiple fields?

Question

Ruby on Rails

Hello. There are 2 tables.
Test1 ( id1:integer, id2:integer, id3:integer, enabled:boolean )

Migration for Test1

class CreateTest1s < ActiveRecord::Migration[5.1]
  def change
    create_table :test1s do |t|
      t.integer :id1
      t.integer :id2
      t.integer :id3
      t.boolean :enabled

      t.index [ :id1, :id2, :id3], name: :test1s_id1_id2_id3_un, unique: true
    end

    reversible do |direction|
      direction.up {
        execute <<-SQL.squish
        INSERT INTO test1s ( id1, id2, id3, enabled )
          VALUES
            ( 1, 1, 1, true ),
            ( 1, 1, 2, true ),
            ( 1, 1, 3, true ),
            ( 1, 2, 1, false )
        SQL
      }
    end
  end
end

Test2 ( id1:integer, id2:integer, id3:integer, id4:integer, count:integer )

Migration for Test2

class CreateTest2s < ActiveRecord::Migration[5.1]
  def change
    create_table :test2s do |t|
      t.integer :id1
      t.integer :id2
      t.integer :id3
      t.integer :id4
      t.integer :count
    end

    reversible do |direction|
      direction.up {
        execute <<-SQL.squish
          ALTER TABLE test2s
            ADD CONSTRAINT test2s_id1_id2_id3_fk FOREIGN KEY (id1,id2,id3)
            REFERENCES test1s ( id1,id2,id3 )
        SQL

        execute <<-SQL.squish
          INSERT INTO test2s ( id1, id2, id3, id4, count )
            VALUES
              ( 1, 1, 1, 1, 1 ),
              ( 1, 1, 2, 2, 4 ),
              ( 1, 1, 2, 5, 6 ),
              ( 1, 2, 1, 1, 3 )
          SQL
      }
    end
  end
end

Test2 subordinate Test1 by fields (id1,id2,id3)
How do I properly set up a relationship in the model?

class Test2 < ApplicationRecord
  belongs_to :test1, foreign_key: :test2s_id1_id2_id3_fk, class_name: Test1
end

Googled but didn't google anything. Thank you.