When Many-to-Many Through Associations release?

[cuongcua90]

The docs said it is coming soon. while I wait it is released is the alternative way to attack addition attributes?

[devinivy]

I am able to get this mostly working (not fully tested). It's a pretty manual job, and finnicky! For now, don't bother with attribute methods or lifecycle callbacks, as they'll get overwritten. I have an issue about that here: #707. Here's an example. I would love for someone to let me know if there's any easier way! In particular, I'm confused by the via attribute on the through table's foreign keys, and something might not be quite right. I suppose a lookup might happen, as the through table never references the other tables' foreign keys; thus, there's no clear connection between the humanFriends and dogFriends attributes. Nonetheless, it's working!

var Dogs= {
        identity: 'dogs',     
        attributes: {
            humanFriends: {
                collection: 'humans',
                through: 'friendships'
            } 
        }
},

var Friendships = {
        identity: 'friendships',
        attributes: {
            someOtherAttr: 'string',
            dog: {
                columnName: 'dog',
                type: 'integer',
                foreignKey: true,
                references: 'dogs',
                on: 'id',
                onKey: 'id',
                via: 'human'
            },
            human: {
                columnName: 'human',
                type: 'integer',
                foreignKey: true,
                references: 'humans',
                on: 'id',
                onKey: 'id',
                via: 'dog'
            }
        }
}

var Humans = {
        identity: 'humans',
        attributes: {
            dogFriends: {
                collection: 'dogs',
                through: 'friendships'
            } 
        }
}

[mewben]

Not working for me... TypeError: Cannot read property of 'via' of undefined :(

[devinivy]

@mewben, do you get that error right when waterline is initialized / when your sails lift (if you're using sails)? And which version of waterline are you using? I'd love to find a reproducible way to hack through-associations into working correctly!

[mewben]

I'm using sails v0.10.5... sails lifts okay though.. the error triggers when using .add() in the association and then .save() to commit...

[devinivy]

@mewben using that exact example I gave you above, this is working for me. I'm on waterline 0.10.12 using sails-disk. Perhaps if you follow that error's stack trace, we can figure out if it's simply the version of waterline that you're running.

[mewben]

I'm using the waterline v0.10.9 which sails v0.10.5 is using.. I guess I'll have to wait for sails to bump their waterline dependency then...

[dmarcelino]

I've tried a similar code with v0.10.12 and it worked for me, thanks @devinivy.

[NiclasLindqvist]

The issue title did not correspond to the question in the issue body here.. Is there a date for the 'official' associations-attributes implementation? Or is it just the docs that are missing, and @devinivy has the right way to do it?

[devinivy]

I'm not sure about an official date/release– sadly it doesn't seem to be in the 0.11 milestone: https://github.com/balderdashy/waterline/milestones/0.11 . My method works, but it's undocumented. If you peek through the waterline-schema codebase you'll see some references to the through keyword. I expect when this is officially supported that the model configuration on the junction-collection will be simpler than my example above.

[dmarcelino]

@NiclasLindqvist, the waterline-adapter-tests give a hint as to what may be the right way to describe many-to-many through associations in the future. Check hasManyThrough.venue.fixture.js (join table) and hasManyThrough.team.fixture.js. This approach works for joins (relevant test) but it seems to fail when creating an association. Using @devinivy's method does work for both creating associations and joins which makes me think that many-to-many through associations will be officially supported soon (fingers crossed).

[kirkaleze]

@devinivy what statement are you using to populate your join model once humans and dogs are populated? Also, is declaring the variables necessary?

[devinivy]

@kirkaleze declaring the variables is merely for illustrative purposes. The populate query would then look quite normal:
DogCollection.find().populate('humanFriends').exec(function(err, dogs) {});

But you should also be able to query the junction table:
FriendshipsCollection.find().exec(function(err, friendships) {});

[dmarcelino]

It would also be good to be able to store data in the join table (association) while saving. Quoting @elennaro (#957):

title: extra fields on many to many relationships join table
It's a widely used approach when we save extra data with relation.
Like quantity for sold products in order-product relation.
Like expiration on user-post-ban. etc. etc.

[devinivy]

Yes, indeedy! Info on the join table should also be included appear when populating records during a find.

[elennaro]

Sorry for a noob question, but is there any manual way to overwrite find for humans somehow to get both Dogs and identity field from friendship?