How to add to a list type in Python Eve without replacing old values

Question

I have a very similar setup to the person in this question: How do I update a list data_relation in Python Eve with a users resource and a friends sub-resource of list type.

users = {
    …
    ‘friends’: {
    'type': 'list’, 
        'schema': {
        'type': 'objectid’, 
        'data_relation': { 
            'resource': 'users’ 
        } 
    }
 }
},

However, when I try to add a new value to the friends list, the other values in the list get replaced by the new value. How do I add a single value to the list and keep the old values?

GET /users/5522987f893e3902048c55ff

{
"_updated": "Wed, 15 Apr 2015 17:22:07 GMT",
"_created": "Mon, 06 Apr 2015 14:30:23 GMT",
"_id": "5522987f893e3902048c55ff",
"friends": [
    "552e9eb0893e391063045edc"
]
}

PATCH /users/5522987f893e3902048c55ff
{"friends": [“550f288d893e390204b0a5ac”]}

RESPONSE:
{
"_updated": "Wed, 15 Apr 2015 19:38:06 GMT",
"_created": "Mon, 06 Apr 2015 14:30:23 GMT",
"_status": "OK",
"_id": "5522987f893e3902048c55ff"
}

GET /users/5522987f893e3902048c55ff

{
"_updated": "Wed, 15 Apr 2015 19:38:06 GMT",
"_created": "Mon, 06 Apr 2015 14:30:23 GMT",
"_id": "5522987f893e3902048c55ff",
"friends": [
    "550f288d893e390204b0a5ac"
]
}

I have also tried PUT, but it replaces the list with the new value as well.

EDIT: I just tried using POST.

POST /users/5522987f893e3902048c55ff/friends
{"552e9eb0893e391063045edc"}

RESPONSE:
{
"_status": "ERR",
"_error": {
    "message": "The requested URL was not found on the server.  If you entered the URL manually please check your spelling and try again.",
    "code": 404
}
}

AND

POST /users/5522987f893e3902048c55ff
{"friends": ["552e9eb0893e391063045edc"]}

RESPONSE:
{
"_status": "ERR",
"_error": {
    "message": "The method is not allowed for the requested URL.",
    "code": 405
}
}

Answer 1

POST will allow you to add a new document to the users endpoint. If it is returning a 405 then you most likely need to enable the POST method by adding it to the RESOURCE_METHODS list, as all endpoints are read-only by default; see docs (alternatively you can only enable POST for the individual endpoint by adding the method to the local resource_methods instead.)

PATCH allows for the replacing of individual fields (as opposed to PUT, which will replace the whole document). So with PATCH you can atomically update the friends list by replacing it with a new value (a new list ObjectIds in your case), but you cannot push a single new ObjectId inside the existing list, I am afraid.