Use only lesson exercises in user_stats activity

[luchotc]

:dart: Goal

Only take into account lesson related exercises

:memo: Details

There is a problem with the current implementation of user_stats activity. It's using organization.exercises which is the fastest way of accessing organization exercises but it has the problem that it's taking into account exams and complements as well, because it's being joined through usages.

There are two alternatives:

• The easy one is the one I've implemented. Using organization.book.exercises. This one has the problem that the query is heavier. It's going through the entire content tree to get the exercises.
• The correct one would be to implement an organization method that gets book exercises through usages (using parent_item_type = 'Lesson'). I think in fact, that we could just update exercises organization method instead of creating a new one. It's being used in places where it makes sense to access only book exercises.

What do you think @julian-berbel @felipecalvo?

:back: Backwards compatibility

100%

[felipecalvo]

Well, you made it pretty clear... I guess the second one is the "right" solution (as long as no current use of Organization::exercises requires complements and exams), but I don't know if it demands much extra work.

This is the kind of situation where it's very unlikely that we change it or improve it at a later point, I think.

[luchotc]

Great :smile: