Special method/mixin "add" for BelongsToMany association overrides record instead of creating a new one

[rdsedmundo]

Issue Creation Checklist

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Bug Description

Reproducible Example

Here is the link to the SSCCE for this issue: rdsedmundo/20240123-sequelize-add-mixin-method-bug@af84fd6

What do you expect to happen?

A new record should have been created, especially because another field of the junction table is passed—although to be honest in any case I think I'd expect what is happening without an unique constraint being defined.

Setting through.unique to false doesn't change anything.

What is actually happening?

The third test is executing this:

[Sequelize] Executed (default): UPDATE FooBarAttachments SET restrictionId=$1 WHERE fooName = $2 AND barName = $3; {"$1":3,"$2":"TS foo","$3":"TS bar"} [Elapsed time: 0 ms]

Environment

• Sequelize version: 6.11.0
• Node.js version: 20.9.0
• If TypeScript related: TypeScript version: 5.1.6
• Database & Version: Postgres 16.1
• Connector library & Version: 8.9.0

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

• Yes, I have the time and I know how to start.
• 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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