How to implement a partial resource rest api? - performance

Question

In order to limit the size of my REST API answers, I want to implement the Google performance tip: using the fields query string parameter to do partial resources.
If I have a full answer GET https://myapi.com/v1/users
[
{
"id": 12,
"first_name": "Angie",
"last_name": "Smith",
"address": {
"street": "1122 Something St.",
"city": "A city"
..and so on...
}
},
... and so on
]
I will be able to filter it GET https://myapi.com/v1/users?fields=first_name
[
{
"id": 12,
"first_name": "Angie"
},
... and so on
]
The concept is pretty easy to understand, but I can't find an easy way to implement it!
My API resources are all design the same way:
use query string parameters for filtering, sorting, paging.
call a service with that parameters to do a SQL request (only the WHERE condition, the ORDER BY condition and the LIMIT are dynamic)
use a converter to format data back to JSON
But when using this new fields parameter, what do I need to do? where do I filter the data?
Do I need to filter only the JSON output? But I will make (in that example) an unwanted JOIN query on address table and fetch unwanted fields in the users table.
Do I need to make a dynamic SQL query to fetch exactly the requested fields and add the JOIN only when the end user need it? Then the converter will have to be smart to convert only the available fields in the SQL query.
In my opinion, this second solution will produce a code extremely dynamic, extremely complex and difficult to maintain.
So, how do you implement such REST API with partial resource feature? What are you best practice in that case?
(I'm a PHP developer, but I don't think it's relevant for that question)

Answer 1

If your backend is doing
GET https://myapi.com/v1/users
which results in SQL:
select * from users
which you then turn into JSON, can you not just do:
GET https://myapi.com/v1/users?fields=first_name,surname,email
get all the required fields (rough idea of PHP implementation):
$fields = split(",", $_GET["fields"]);
$sql = "select ";
foreach ($fields as &$field) {
// do a check to see if the field is ok first...
if (checkField($field)) {
$sql += field + "," // deal with commas
}
}
$sql += " from users";
to build SQL like:
select firstname,surname,email from users
and turn that limited dataset to JSON?