Building Data-Driven Web Apps with Pyramid and SQLAlchemy Transcripts
Chapter: Using SQLAchemy
Lecture: Concept: Relationships
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We saw relationships are extremely powerful
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and let us imagine as if our code and data
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were just connected almost hierarchically
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the way we would in a regular Python program connect it
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not split apart like we do in a database.
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The way we define these
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we had an orm.relationship field to the class.
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So here we have our releases
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say the relationship is against the release type
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that we're speaking in terms of SQLAlchemy entities
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not database and then we're going to do an orderby.
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This could be a single thing or a list.
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Probably an integer actually
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and so we're going to pass that in.
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And then we're going to say it back-populates package.
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What does that mean?
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If we want this relationship to work in both directions.
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So we have a package we can see the releases
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and if we have an individual release
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we can see the single package that it corresponds to.
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So over in the release we're going to do the package ID value
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to actually store the relationship
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like we would store any value that's a string or an integer
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whatever it corresponds to in the other field.
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And then we're going to say this is a foreign key
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and in the foreign key part we speak in terms of database
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packages.id
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But then we also would like to establish that relationship
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so we say there's orm relationship
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for the package type form released back to the package.
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And it back populates releases and it's called package.
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So you can see the symmetry here.
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Not too hard to set this up, once you have an example.
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Put them side by side, you go okay
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here's where I fit all the pieces for my particular data set
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and then you saw that when we load a package
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it doesn't actually load the releases
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unless we interact with it.
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So if we touch it, it would go back to the database
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and do the query.
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We also saw that if we create new releases
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and put them into the release package.releases
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what becomes a list and commit those changes
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that will actually insert the releases.
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We work on the same but in reverse as well
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if we had said a package on a release.
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So it's sort of bi-directional in that sense.
