Include related model in an hasOne relation - loopbackjs

Question

I have two models: Account and Customer both having an email address.
An Account can exist without a Customer and a Customer can exist without an Account.
However, an Account should return the related Customer record if this exists.
I was thinking about doing this by creating a hasOne relation on the Account using the unique identifier available in both records (the email address) as foreignKey.
Unfortunately this is not working.
These are my models:
Account
...
"properties": {
"username": {
"type": [
"string"
]
},
"email": {
"type": "string"
}
},
"validations": [],
"relations": {
"customer": {
"type": "hasOne",
"model": "Customer",
"foreignKey": "email"
}
}
...
Customer
...
"properties": {
"name": {
"type": [
"string"
]
},
"email": {
"type": "string"
}
},
"validations": [],
"relations": {}
...
By calling /api/account?filter={"include": ["customer"]} I don't get any additional information.
I don't understand if the problem is the foreignKey or the relation.

Answer 1

You could use an afterRemote hook to do the marshaling just before returning the requested instance.
However this won't be automatic, i.e. you still need to provide some sort of id to link the two instances together. In your case, if the email is such an id, then you would just search for a Customer instance with the same email as the Account instance.
The advantage is that you don't need to provide any extra filters or anything else to your query.
e.g.
Account.afterRemote('find', function(ctx, modelInstance, next) {
// Here you can check if the Account instance has a Customer instance
// via a regular find or findById, and if you do find the related instance
// you can add the data to ctx.result, which is the object that will be returned.
Customer.find({where:{email: modelInstance.email}}, addCustomerDetails);
function addCustomerDetails(err, linkedCustomer) {
// Add the Customer to the Account instance here
ctx.result.customer = linkedCustomer;
next();
}
});
And of course, you can do the same in the Customer afterRemote hook, but instead searching for the linked Account instance email.

Answer 2

Your models are defined well.
Be sure you have customer instance with existed email in db.
And the correct for of rest api calls is : /api/account?filter[include]=customer
UPDATE
Loopback overwrite the type of email because of the relation. hasOne relation should be setup over id foreign key not any other fields.
So if you want to fix the problem, you need to add below to properties section of account definition :
"id": false,
"email": {
"type": "string",
"id": true
}

Answer 3

The foreignKey field is just an alias for the relation. Having an email property and setting email as foreignKey does not create any sort of link between the two.
Then, it's simply a matter of using the REST API to instanciate the models, setup the relation and fetch the data
create an account
POST api/accounts/
{
"email": "account#bar.com"
}
create a related customer
POST api/accounts/1/foreignKey
{
"email": "customer#bar.com"
}
Fetch the accound and include the related customer
GET api/accounts/1?filter[include]=foreignKey