relations.md

Model relationships

We learned how Models and Collections work in a previous guide and how to use them to store data in the cloud. Another important concept is relationships between models. This is often used when modeling data in applications.

If we imagine an app where we have a couple of different collections, say a Post collection with blog posts and a Comment collection with comments on these post. These models need to be related and we need to be able to query these relations.

One-to-many relationships

This type of relationship is often referred to as a one-to-many relationship, in that one blog Post will have many Comments, but each comment only belong to one post. This is done using a special type of property called a Pointer.

A Pointer property can reference another model of a certain class via it's Id. In this case we want to reference the Post that is the owner of a Comment. So in the Comment class we create the Owner property, and give it a Pointer type and specify that it should point to Post models.

Now when you create a new Comment model you can simply pass the Id of the Owner model, it should be a Post model that is the owner.

When you have a Post model and you want to query all Comments on the post you need to use the advanced filter format in the Query Collection node. The filter you need is the following:

where({ 
    {Owner:{pointsTo:Inputs.PostId}}
})

What this filter does is that it will only select the models where the Owner property contains a specific Id, something that we will provide to the Query Collection via an input connection.

What you get is really a back reference from the Comment, it has a Pointer property that points to it's owner and that is used to filter out the Comments for a given Post when needed.

You can inspect your relationships in the data browser, if you go ahead and open the Dashboard for the cloud services and find the Comment table. There you can see the model Id that the pointer currently points to, and you can click it to jump to that particular model.

Many-to-many relationships

Let's say that we introduce a new model called Group, and a Post can be part of many different Groups. In this case we cannot use the backwards pointing mechanism from the previous example. Instead we need to use a concept called Relation. You need to start by creating a new property of the Group model that have the type Relation and like pointers you need to specify the Type and give it a name.

?> So in this case it becomes a forward reference, the Group has the relation property.

When you have a Relation property on a model you will get some additional inputs that you can use in to manage relations.

Just like you would use an Array you can provide an Id to a model (it needs to be from the Post collection) and then send a signal to Remove or Add in the relation group to remove or add a relation to the model.

In the same manner as with Pointers you can go to the Dashboard of the cloud services and find the relations of your models in the table. You can click View relation to get a table of the relations for this particular model.

Finally you need to be able to query the relations. So let's say that we have a Group model by Id and we want to find all Posts that are related to that group. For this you will also need the advanced filter setting of the Query Collection. You would create a Query Collection node and make sure it is set to the Post collection. Then put in the following advanced filter.

where({ 
    relatedTo: {id:Inputs.GroupId, key:'posts'}
})

The above filter will make sure to only return the Post models that are related to the provided Group via the GroupId input and that are related via the posts property of the Group. This is how you might hook it up in the node graph.

If you want to make the inverse query, that is you have a Post model (you have the Id of a Post model) and you want to find all Groups that it belongs to you would use this advanced query in a Query Collection that is set to the Group collection.

where({ 
    posts: {pointsTo:Inputs.PostId}
})

As you can see above you must also make sure that you use the correct relation field, in this case posts on the Group collection.