Bug with mixins when using with custom unique constraint in M:N relation

[JaoodxD]

Issue Creation Checklist

• [x] I understand that my issue will be automatically closed if I don't fill in the requested information
• [x] I have read the contribution guidelines

Bug Description

The problem is that model mixins incorrectly interacts with unique constraints.

Let's say we have model A and B.

We need M:N relation between them. We establish it throuh junction table C.

This relation should contain additional boolean field flag. At our case we need to extend default unique constraint with this flag i.e. uniqueness should be determined by three fields ( Aid, BId, flag).

To do this we have perform 2 steps:

1. Turn off default constraint by passing unique : false inside belongsToMany relation definition.
2. Define our own unique combined key with both AId, BId and flag.

Now we suggest that we can relate A with B twice (but with different flag value). It works fine if we directly add two records inside C :

const a = await A.create();
const b = await B.create();
await C.create({ 'Aid': a.Id, 'Bid': b.Id, 'flag': true });
await C.create({ 'Aid': a.Id, 'Bid': b.Id, 'flag': false }); // <- flag is different
console.log(await C.count()); // <- will display us 2 as expected

But if we do the same with the help of mixins addB we'll get different output:

const a = await A.create();
const b = await B.create();
await a.addB(b, { through: {flag: true } });
await a.addB(b, { through: {flag: false } }); // <- same different values
console.log(await C.count()); // will display us 1 instead of 2.

Reproducible Example

Here is the link to the SSCCE for this issue: https://github.com/JaoodxD/sequelize-sscce-m-m-bug

What do you expect to happen?

When we turn off the unique constraint inside belongsToMany declaration mixin addB should create two different records inside junction table C.

What is actually happening?

Mixin addB still check only (Aid, BId) constraint and update previous record.

Environment

• Sequelize version: 6.21.3
• Node.js version: 18.5.0
• Database & Version: PostgreSQL 14.3
• Connector library & Version: [email protected]

Would you be willing to resolve this issue by submitting a Pull Request?

• [ ] Yes, I have the time and I know how to start.
• [x] Yes, I have the time but I will need guidance.
• [ ] No, I don't have the time, but my company or I are supporting Sequelize through donations on OpenCollective.
• [ ] No, I don't have the time, and I understand that I will need to wait until someone from the community or maintainers is interested in resolving my issue.

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[JaoodxD]

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