Building data-driven web apps with Pyramid and SQLAlchemy Transcripts
Chapter: Modeling data with SQLAlchemy classes
Lecture: Concepts: Modeling
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Before we actually start using SQLAlchemy to
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insert data and query data and so on,
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Let's talk about some of the core concepts we've seen
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and some of the fundamental building blocks
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for modeling with SQLAlchemy.
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So we started with the SQLAlchemyBase.
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Remember, the idea was every class we're going to
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store in the database derive from this
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dynamically defined SQLAlchemyBase class.
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You can call it whatever you want.
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I like SQLAlchemyBase, but there's other
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you know, it's just a variable
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name it as you like.
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So we want to create this singleton base class
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to register the classes and types that go in the database.
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Remember, there's one and only one instance
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of this SQLAlchemyBase shared across all of the types
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per database. So for example, we're going to have
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a package, a release, and user
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they all derive from this one, and only one
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SQLAlchemyBase type here.
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To model data in our classes, we put a bunch of
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class level fields here:
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ID, summary, size, homepage, and so on.
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And each one of them is a column.
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SQLAlchemy.column and they have different types
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like integer, string, and so on.
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We can see some of them are primary keys
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and even if it's an integer
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they can even be auto-incrementing primary keys
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which is really really nice.
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And we can also have relationships
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like we do between package and releases.
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One really nice feature of databases is
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they have default values.
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We saw with our auto-incrementing ID
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our primary key we don't have to set it
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the database does that for us.
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So here we can pass datetime.now
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the function not the value, the function
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and then it's going to call that function, now
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whenever a row is created and set that value
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to be, well, right now. That's super nice.
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We can also do that up here with more complex expressions.
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So in the bottom one we've literally passed
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an existing function, datetime.now
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but above we wanted to define this default behavior
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in a more rich way.
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So we're passing our very own Lambda expression
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that takes the uuid for identifier
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converts it to a string and then drops the dashes
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that normally separate it
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into just one giant scrambled alphanumeric soup thing.
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You can create these default values by passing
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any function a built in one or one of your own making.
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You also want to model keys and indexes.
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So primary keys automatically have indexes
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we don't have to do anything there.
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That's got a uniqueness constraint
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as well as a indexes. This created one
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maybe we want to sort by the newest users
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for example. Well if we're going to do that
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we very much want to put an index on that.
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As I pointed out, indexes can have
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tremendous performance benefits.
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It's totally reasonable to have a
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thousand times difference performance in a query
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if you have tons of data
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on whether you have an index or not.
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Indexes do slow write time
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but certainly, in this case the rate of user creation
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versus querying and interacting with them is
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you know, it's no comparison, right?
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We're creating far fewer users probably
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than we are querying or interacting with them.
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We could also specify uniqueness.
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We didn't do that in our example.
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We can say this email
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we can't have two users with the same email.
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You know, emails are very often used to like
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reset your password.
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And if you have two users
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who's going to get their password reset?
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All of 'em? One of 'em?
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Who knows, none of 'em?
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So you might want to say there's a uniqueness constraint
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on the email to say
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"Only one user gets to use particular email"
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and that's super easy to do by just
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saying unique equals True.
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Finally, once all of the modeling is done
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we have to actually create the tables
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and it turns out that that's super easy.
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We import all the packages.
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Get the connection string
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and we create an engine based on the connection string
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and then we just go to SQLAlchemyBase
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to it's metadata and say
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create_all and pass the engine.
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Boom, everything is done.
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Remember though, this only creates new tables
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it does not modify existing ones.
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So if you need to modify it
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wait till we get to the Alembic chapter
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the migrations chapter or do it yourself.
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Or, you know, if you're just in development mode
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maybe deleting it and just letting it recreate itself.
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That might be the easiest thing
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that's what we did.
