SQL - REST - Storing data and retrieving it using friendly IDs - sql

Question

UPDATE 09/06/2018
My models looks like this:
public class Activity
{
public int Id { get; set; }
public List<Additive> Additives { get; set; }
}
public class Additive
{
public int Id { get; set; }
public int ActivityId { get; set; }
public Activity Activity { get; set; }
}
Original Question
I'm developing a RESTful API and i want to do it the cleanest way possible.
Currently i'm working on data retrieving. So i got this
If i create a new resource called activity and it has an array of objects, lets say (hardcoded, not representative):
{
"name": "act1",
"objects": [
{ "obj1":"val1" },
{ "obj2":"val2" }]
}
I'll could add or delete objects from my array.
Now, my question is, if i want to access them with an url like this:
Api/activity/1/objects/2
Which is the correct way to tell MySQL that this object is my 2th object of the list?
Or maybe i should select ALL objects of Act1 on the backend and filter them to retrieve just the 2th one.

Answer 1

It's pretty important that resources in your REST service have stable urls. So to figure out what 'second' is, you really need to think about what it means to be second in the list.
Are you ordering by something? If so, it's probably a bad idea to use this structure because adding a new entry can change the order and what was 'second' before might no longer be in the future.
The best thing you can do is add some kind of new id for these entries so accessing a specific resource by uri will always return that resource (or 404, etc).

Answer 2

I finally found the answer i were looking for.
mysql two column primary key with auto-increment
This is a MyISAM engine feature.
For MyISAM and BDB tables you can specify AUTO_INCREMENT on a secondary column in a multiple-column index. In this case, the generated value for the AUTO_INCREMENT column is calculated as MAX(auto_increment_column) + 1 WHERE prefix=given-prefix. This is useful when you want to put data into ordered groups.
In my case, this query does the job
CREATE TABLE additive (
id INT NOT NULL AUTO_INCREMENT,
activity_id INT NOT NULL,
other_column VARCHAR(30) NOT NULL,
PRIMARY KEY (activity_id, id)
) ENGINE=MyISAM;