Building Data-Driven Web Apps with Pyramid and SQLAlchemy Transcripts
Chapter: Modeling data with SQLAlchemy classes
Lecture: Connecting to our 'database'
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We define our class that maps the database
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we have our SQLAlchemy base class.
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Next thing to do is actually create
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that connection and configure the database.
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So again, a new file to organize that code.
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Call it DBSession.
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So this is going to serve two purposes
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it's going to initialize the access to the database
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and it's also going to manage this overall session
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this unit of work design pattern so hence the name.
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We're going to come down here and define a class called DBSession
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and we're going to give it two pieces of information.
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We're going to have a factory and we're going to have a engine.
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The engine is going to manage the
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type of database we're talking to
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and the connection and the connection pooling.
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The factory is going to be the thing
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that creates sessions on demand, okay and
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the rule about these is there should be
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one and only one of them per connection
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okay so that's basically a singleton
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unless you're having multiple connections
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to multiple databases, in which case
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you got to manage that slightly
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differently than I'm showing you here.
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You want to have a static method called global_init
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and then we'll just make sure we
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call this once and only once, so we'll say if
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DBSession.factory and it's already been called
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no problem
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So we're also going to need to take a DB file to the string.
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What we're going to do is we're going to use SQLLite for this.
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That means when you set up this code and you run it
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you have no database to configure
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install, manage, etc.
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In production you probably if you're
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using SQLAlchemy you'll want to use some like Postgres
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or something along those lines
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that's just changing the connection string
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should be no problem but for here
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we're going to let them pass in a file
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and that file is going to represent
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just the actual SQLAlchemy file on disc.
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That's all you need for a SQLAlchemy connection.
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So it's probably prudent to do a little test here.
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Alright, now we know we have at least
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something that could be a file.
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Now let's define the connection string.
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SQLite always looks like this.
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sqlite:///<DB file>
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and I always like to print this out
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like here's my connection string.
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If this had a username password in it
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maybe I wouldn't exactly print that
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I'd print you know maybe not the password for sure
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but let's just go ahead and print.
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Connecting then like this connecting
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to DB at the connection string
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and then what we're going to do is
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we're going to create the engine.
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So we'll say engine equals SQLAlchemy.create_engine.
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Now it takes *args **kwargs which means
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thanks for the help, nothing sadly
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but what we can do is we'll pass
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the connection string here and then echo equals
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What do we put here, let's put false for now.
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If you would like to see every operation
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every bit of SQL that is sent to the database
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as it happens, print it out, change that to True
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it's a really nice debugging technique
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to see what's happening.
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Alright so because we want one and only one
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of those we're going to store that here
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and we also need while we're at it
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let's go ahead and create this factory
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and it'll just round out this thing here
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so we'll say DBSession.factory
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say SQLAlchemy.session.orm
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Better import that at the top too
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.sessionmaker
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say bind equals engine
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I think we're in good shape.
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Okay so this will allow us to
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initialize the connection and create this factory
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and then later when we want to actually
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create the factories we'll see there's
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a couple of evolutions or steps of improvement
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that will apply to this factory thing here as well
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but for now this is the sort of standard starting place
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so what we do is we come up with a connection string
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we create the engine based on that
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and we hold onto it, one and only one of them
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same thing, we create the factory based on the engine
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and this is going to manage both the connection string
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as well as the dialect, the type of database
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we're talking to because of that right there.
