[v5] Cannot populate multiple relations to the same table using the REST API and Document Service API

[razvanilin]

Bug report

• Node.js version: 20.10.0
• NPM version: 10.2.3
• Strapi version: v5-beta.7
• Database: MySQL
• Operating system: MacOS
• Is your project Javascript or Typescript: Javascript

Describe the bug

I have a table that has two different relation fields with the same table. So in table Property I have the following:

• contracts is a field with the belongs to many relation
• active_contract is a field with has one relation

The problem is that when I use the REST API and include the populate query param, only the contracts gets populated even though I indicate that I want active_contract too. active_contract always returns as null.

When using the new Document Service API directly in the controller with the same params, active_contract gets populated and contracts is []

The only thing that worked is to write a custom controller for the find and findOne methods to return the results with the Entity Query API. So it seems that in v5, I cannot rely on the REST API directly. This definitely feels like a bug

Steps to reproduce the behavior

1. Create table property
2. Create table contract
3. Add two relation fields to property called contracts (belongs to many) and active_contract (has one)
4. Enter some dummy data
5. Try the REST API with the populate query params and it will not return both of the relations

Expected behavior

I want both my relations populated when I make a REST API call or use the new Document Service API

Screenshots

Code snippets

❌ REST API Example on the client that doesn't populate active_contract:

const url = `${API_URL}/properties`;
    const method = "GET";
    const headers = {
      "accept": "application/json",
      "authorization": `Bearer ${getCookie(AUTH_TOKEN)}`,
    };

    try {
      const response = await axios.get(url, {
        method,
        headers,
        params: {
          filters: {
            team: {
              id: { $eq: teamId }
            },
          },
          populate: {
            image: true,
            active_contract: true,
            contracts: {
              populate: ["renters"],
            },
          },
        },
      });

      return response.data;

❌ Document Service API where this time contracts returns an empty array and active_contract is populated:

const entities = await strapi.documents('api::property.property').findMany({
      filters: {
        team: {
          id: { $eq: teamRecord.id },
        },
      },
      populate: {
        image: true,
        contracts: {
          populate: ["renters"],
        },
        active_contract: true,
      },
    });

✅ And the Query Engine API that works:

const entities = await strapi.db.query('api::property.property').findMany({
      where: {
        team: {
          id: { $eq: teamRecord.id },
        },
      },
      populate: {
        image: true,
        contracts: {
          populate: ["renters"],
        },
        active_contract: true,
      },
    });