[MUSIC]

Hi, I'm Kenneth, the Python teacher here at Treehouse.

We've been using Django's ORM in all of our agenda courses so far.

We used it to get single records out that matched a certain primary key.

And we used it to get all of the records for

a certain model like when we made our list of courses.

We also used it when we were making model formsets and needed a set of blank forms.

Let's do a mini review.

We'll use our course model.

If we want all of the courses that are saved in the database,

we would use course.objects.all.

If we didn't want any of them but we wanted a query set,

we'd use course.objects.none.

And if we needed just a single course,

we would use the get method to pull it out using a comparison.

Of course, if we think that course might not exist,

we'd use the handy get object or 404 function.

The last thing we used the ORM for was creating and saving records.

We used the create method to make a new instance and the save method to save

changes to an existing one or to insert an in-memory instance into the database.

But even with all of that,

there's still a lot about the ORM that we haven't looked at.

I mean, heck, I haven't even given you a formal definition for what a query set is.

A query set, at least in Django's terms,

is a collection of records returned from the database.

You might be expecting the collection to be a set since that's in the name or

at least unique but that's not always the case.

Anything that could come back from a plain old SQL query can be in a query set.

Query sets are also lazy.

Lazy is a word you'll hear a lot in Django, actually.

When we use lazy in a programming concept, we usually mean that whatever work

the lazy thing is going to do won't be done until we use the lazy thing.

For instance, query sets exist only in memory.

They don't hit the database at all until you consume them with Lynn, a slice,

iteration or some other options.

I'll include a link to more information about this in the teacher's notes.

Most of this course will be dealing with the model layer of Django.

Things like, well, models and query sets.

So in the next video, let's check out the current state of our models, our data,

and make a couple of small changes.