Support for a n:m table where both are polymorphic

Is your feature request related to a problem? Please describe.

The sequelize functions don't work well with a n:m table where both sides to it are polymorphic.

For example, let's extend the example given in the docs here: http://docs.sequelizejs.com/manual/associations.html#n-m

Let's say that we don't only have 'tags' but we also have 'comments'. So, the current situation is that a post/image can have many tags/comments and vice versa and we want a joint table to represent that. Now both pieces are polymorphic, which is different from in the example.

The issue now is that say I want to associate an image instance with a list of tags already in the tags table:

imageInstance.addTags(tagIds);

When inserting into the join table, it won't grab the the column/scope defined in the Tags table. The SQL generated looks like: INSERT INTO <jointTableName> (postOrImage, postOrImageId, tagOrCommentId) VALUES...

Describe the solution you'd like

The above should do: INSERT INTO <jointTableName> (postOrImage, postOrImageId, _tagOrComment_, tagOrCommentId) VALUES...

A workaround that worked was working was:

imageInstance.addTags(tagIds, _{ through: {tagOrComment: 'tags' }}_);

But that came with another issue - Mainly that the 'check' Sequelize does before inserting into a joint table to ensure it's unique, also does not check the tagOrComment column, so if I then ran this command:

imageInstance.addComments(commentIds, _{ through: {tagOrComment: comments }}_);

It would just check if the 'postOrImage', 'postOrImageId', and 'tagOrCommentId' are unique and not the 'tagOrComment'. This is an issue if the tag/comment share an ID, because if they did, it performs an update thinking it's the same record.

Why should this be in Sequelize

Because it's pretty natural.

sequelize / sequelize

Support for a n:m table where both are polymorphic #10854