Loopback.io belongsTo in REST

I'm a bit confused about the belongsTo relation function of loopback.io

So let's take the following example:

I have a model called Project which has a relation to a Customer object. So a Project belongs to a Customer.

This is my Project model

{
  "name": "Project",
  "plural": "Projects",
  "base": "PersistedModel",
  "strict": false,
  "idInjection": false,
  "options": {
    "validateUpsert": true
  },
  "properties": {
    "title": {
      "type": "string",
      "required": true
    },
    "description": {
      "type": "string"
    },
    "dateCreated": {
      "type": "date"
    }
  },
  "validations": [],
  "relations": {
    "customer": {
      "type": "belongsTo",
      "model": "Customer",
      "foreignKey": "customerId"
    }
  },
  "acls": [],
  "methods": {}
}

So when I run the app and I go to http://0.0.0.0:3000/explorer I can see the API. But When I go to project I only see

 GET /Projects/{id}/customer     Fetches belongsTo relation customer.

I was also expecting other functions like

 POST /Projects/{id}/customer 
 DELETE /Projects/{id}/customer

Why are they not available here? Or how can I set the customer for a project via REST API?

Solution

Let's first understand belongsTo relationship, to understand why only GET rest endpoint is created:

GET /Projects/{id}/customer     Fetches belongsTo relation customer.

And why Loopback didn't created the below links:

POST /Projects/{id}/customer 
DELETE /Projects/{id}/customer

belongsTo relationship creates a one-to-one relation between two model instance. It is used to provide ownership of one instance to another instance. In your case, project model instance belongs to a customer model instance. Now since project belongs to customer, therefore customer owns the project instance and so explanation for the rest endpoints.

GET /Projects/{id}/customer     Fetches belongsTo relation customer.

since customer can have project instance, therefore above is valid as customers can be fetched for projects.

POST /Projects/{id}/customer
DELETE /Projects/{id}/customer

since customer don't belong to a project rather has project, above rest endpoints doesn't make sense as projects creating or disowning its owner(customer).