Using databases

– OK, what if I need to use a database in my application?
– you can use the included ORM

Emmett comes with an integrated ORM based on pyDAL, which gives you the ability to use a database in your application writing simple Python code without worrying about queries and specific syntax of the database engine you want to use.

Thanks to this database layer, you can write the same code and use the same syntax independently of which of the available adapters you want to use during development or when you're deploying your app to the world.

This is the list of the supported database engines, where we included the name used by Emmett for the connection configuration and the appropriate driver you need to install, separately from Emmett (just use pip):

The next database engines are included in Emmett but are targeted with experimental support, as they're not officially supported by the Emmett development:

Note:
This list may change, and depends on the engine support of pyDAL. For any further information, please check out the project page.

So, how do you use Emmett's ORM? Let's see it with an example:

from emmett import App
from emmett.orm import Database, Model, Field

app = App(__name__)
app.config.db.uri = "sqlite://storage.sqlite"

class Post(Model):
    author = Field()
    title = Field()
    body = Field.text()

db = Database(app)
db.define_models(Post)

app.pipeline = [db.pipe]

@app.route('/posts/<str:author>')
def post_by(author):
    posts = db(Post.author == author).select()
    return dict(posts=posts)

The above code is quite simple: the post_by() function lists posts from a specific author. Let's retrace what we done in those simple lines:

• we added an sqlite database to our application, stored on file storage.sqlite
• we defined the Post model and its properties, which will create a posts table
• we registered the database pipe to our application's pipeline so that it will be available during requests
• we did a select on the posts table querying the author column

As you noticed, the fields defined for the table are available for queries as attributes, and calling db with a query argument provides you a set on which you can do operations like the select().

In the next chapters, we will inspect how to define models, all the available options, and how to use Emmett's ORM to perform operations on the database.