Preload issue with REL 0.41 / Go 1.22

[nitaigao]

Hi, firstly thanks for this awesome library.

There seems to be an issue with the preloading that has been introduced since rel 0.39.0. The issue causes the nested relations of a preload to be accidentally duplicated.

I discovered this when trying to upgrade from go v1.18 to v.22 which meant that I needed the latest REL to be compatible.

I have tried to investigate but it's not clear to me how to fix the problem.

Versions:

• go v1.22.0
• rel v0.41.0

Here's a quick repro:

BEGIN;

CREATE TABLE questions (
	id SERIAL PRIMARY KEY,
	text TEXT
);

CREATE TABLE answers (
	id SERIAL PRIMARY KEY,
	question_id INTEGER REFERENCES questions(id)
);

CREATE TABLE scheduled_questions (
	id SERIAL PRIMARY KEY,
	question_id INTEGER REFERENCES questions(id)
);

INSERT INTO questions (text) VALUES ('1');
INSERT INTO answers (question_id) VALUES (1);

INSERT INTO scheduled_questions (question_id) VALUES (1);
INSERT INTO scheduled_questions (question_id) VALUES (1);

COMMIT;

package main

import (
	"context"
	"encoding/json"
	"fmt"

	"github.com/go-rel/postgres"
	"github.com/go-rel/rel"

	_ "github.com/lib/pq"
)

type ScheduledQuestion struct {
	ID         int
	QuestionID int
	Question   *Question
}

type Question struct {
	ID      int
	Answers []Answers
}

type Answers struct {
	ID         int
	QuestionID int
}

func main() {
	adapter, err := postgres.Open("postgres://localhost/schedule_dev?sslmode=disable")
	if err != nil {
		panic(err)
	}
	db := rel.New(adapter)

	ctx := context.Background()

	scheduledQuestions := []ScheduledQuestion{}
	db.MustFindAll(ctx, &scheduledQuestions)
	db.MustPreload(ctx, &scheduledQuestions, "question")
	db.MustPreload(ctx, &scheduledQuestions, "question.answers")

	bytes, err := json.MarshalIndent(&scheduledQuestions, "", "  ")
	if err != nil {
		panic(err)
	}
	fmt.Println(string(bytes))
}

output:

[
  {
    "ID": 1,
    "QuestionID": 1,
    "Question": {
      "ID": 1,
      "Answers": [
        {
          "ID": 1,
          "QuestionID": 1
        },
        {
          "ID": 1, // duplicated
          "QuestionID": 1 
        }
      ]
    }
  },
  {
    "ID": 2,
    "QuestionID": 1,
    "Question": {
      "ID": 1,
      "Answers": [
        {
          "ID": 1,
          "QuestionID": 1 
        },
        {
          "ID": 1, // duplicated
          "QuestionID": 1
        }
      ]
    }
  }
]

This result should be:

[
  {
    "ID": 1,
    "QuestionID": 1,
    "Question": {
      "ID": 1,
      "Answers": [
        {
          "ID": 1,
          "QuestionID": 1
        }
      ]
    }
  },
  {
    "ID": 2,
    "QuestionID": 1,
    "Question": {
      "ID": 1,
      "Answers": [
        {
          "ID": 1,
          "QuestionID": 1
        }
      ]
    }
  }
]

[nitaigao]

Both unique question object seem to have a pointer to a single underlying answers collection. I think this is the source of the duplication.

I think it's related to a change in this commit: ea91f66

[Fs02]

Thanks for the detailed description!

I'm able to fix this in this PR: #365
can you verify if it's fixed your issue?

I think the main issue is because pointer is used when the same record is referenced multiple time
without above fix, the following should work correctly:

type ScheduledQuestion struct {
	ID         int
	QuestionID int
	Question   Question // <--- not using pointer here
}

[nitaigao]

I can confirm it's working nicely with the #365 fix

Thanks for looking into it so quickly, I really appreciate it.