Building data-driven web apps with Pyramid and SQLAlchemy Transcripts
Chapter: Using SQLAchemy
Lecture: Concept: Relationships
Login or
purchase this course
to watch this video and the rest of the course contents.
0:00
We saw relationships are extremely powerful
0:02
and let us imagine as if our code and data
0:06
were just connected almost hierarchically
0:08
the way we would in a regular Python program connect it
0:12
not split apart like we do in a database.
0:14
The way we define these
0:15
we had an orm.relationship field to the class.
0:18
So here we have our releases
0:19
say the relationship is against the release type
0:22
that we're speaking in terms of SQLAlchemy entities
0:25
not database and then we're going to do an orderby.
0:27
This could be a single thing or a list.
0:30
Probably an integer actually
0:32
and so we're going to pass that in.
0:34
And then we're going to say it back-populates package.
0:38
What does that mean?
0:39
If we want this relationship to work in both directions.
0:41
So we have a package we can see the releases
0:43
and if we have an individual release
0:44
we can see the single package that it corresponds to.
0:48
So over in the release we're going to do the package ID value
0:52
to actually store the relationship
0:54
like we would store any value that's a string or an integer
0:57
whatever it corresponds to in the other field.
1:00
And then we're going to say this is a foreign key
1:02
and in the foreign key part we speak in terms of database
1:05
packages.id
1:08
But then we also would like to establish that relationship
1:11
so we say there's orm relationship
1:13
for the package type form released back to the package.
1:17
And it back populates releases and it's called package.
1:20
So you can see the symmetry here.
1:22
Not too hard to set this up, once you have an example.
1:25
Put them side by side, you go okay
1:27
here's where I fit all the pieces for my particular data set
1:30
and then you saw that when we load a package
1:33
it doesn't actually load the releases
1:35
unless we interact with it.
1:37
So if we touch it, it would go back to the database
1:39
and do the query.
1:40
We also saw that if we create new releases
1:43
and put them into the release package.releases
1:46
what becomes a list and commit those changes
1:50
that will actually insert the releases.
1:52
We work on the same but in reverse as well
1:54
if we had said a package on a release.
1:57
So it's sort of bi-directional in that sense.
