Building Data-Driven Web Apps with Pyramid and SQLAlchemy Transcripts
Chapter: Modeling data with SQLAlchemy classes
Lecture: Architecture
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Before we actually start writing code for SQLAlchemy
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let's get a quick, 50,000 foot view
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by looking at the overall architecture.
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So when we think of SQLAlchemy
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there's really three layers.
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First of all, it's built upon Python's DB-API.
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So this a standard API, actually it's DB-API2 these days
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but we don't have the version here.
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This is defined by PEP 249 and it defines a way
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that Python can talk to different types of databases
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using the same API.
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So SQLAlchemy doesn't try to reinvent that
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they just build upon this.
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But there's two layers of SQLAlchemy.
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There's a SQLAlchemy core which defines schemas and types
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a SQL expression language that is a generic query language
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that can be transformed into a dialect
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that the different databases speak.
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There's an engine which manages things
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like the connection and connection pooling
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and actually which dialect to use.
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You may not be aware, but the SQL query language
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that used to talk to Microsoft SQL Server
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is not the same that used to talk to Oracle.
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It's not the same that used to talk to Postgres.
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They all have slight little variations
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that make them different.
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And that can make it hard
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to change between database engines.
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But SQLAlchemy does that adaptation for us
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using its core layer.
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So if you want to do SQL-like programming
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and work mainly in set operations, well here you go
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you can just use the core and that's a little bit faster
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and a little bit closer to the metal.
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You'll find most people, though
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when they're working with SQLAlchemy
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will be using what's called an Object Relational Mapper.
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Object being classes, relational, database
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and going between them.
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So what you do is you define classes
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and you define fields and properties on them.
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And those are mapped, transformed into the database
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using SQLAlchemy and its mapper here.
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So what we're going to do is
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we're going to define a bunch of classes
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that model our database.
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Things like packages, releases, users, maintainers
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and so on in SQLAlchemy and then SQLAlchemy
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will actually create the database, the database schema
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everything and we're just going to talk to SQLAlchemy.
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It'll be a thing of beauty.
