Use

Many behaviors, such as the constructor/store, need to have a unique identifier for an instance or instance data. This connection.id method should return that.

Typically, an item's id is a simply property value on the object. For example, "Todo" data might look like:

{_id: 5, name: "do the dishes"}

In this case, algebra's id property should be set to "_id":

var algebra = new set.Algebra({
  set.props.id("_id")
});

connect([...],{algebra: algebra});

However, some data records may have compound ids. For example, "Class Assignment" data may be uniquely identified by a combination of two properties, the studentId and the classId. For this kind of setup, you can provide your own id function as follows:

var customIdBehavior = {
  id: function(assignment){
    return assignment.studentId + "-" + assignment.classId;
  }
}

var classAssignmentConnection = connect(
  [...behaviors..., customIdBehavior],
  {
    url: "/class_assignments"
  }
);