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](https://github.com/web2py/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):

| Supported DBMS | adapter name | python driver |
| --- | --- | --- |
| SQLite | sqlite | |
| PostgreSQL | postgres | psycopg2, pyscopg 3 (experimental), pg8000, zxjdbc |
| MySQL | mysql | pymysql, mysqldb |
| MSSQL | mssql | pyodbc |
| MongoDB | mongo | pymongo |

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

| Experimental support | adapter name | python driver(s) |
| --- | --- | --- |
| CouchDB | couchdb | couchdb |
| Oracle | oracle | cxoracle |
| FireBird | firebird | kinterbasdb, fdb, pyodbc |
| DB2 | db2 | pyodbc |
| Informix | informix | informixdb |
| Ingres | ingres | ingresdbi |
| Cubrid | cubrid | cubridb |
| Sybase | sybase | Sybase |
| Teradata | teradata | pyodbc |
| SAPDB | sapdb | sapdb |

> **Note:**   
> This list may change, and depends on the engine support of pyDAL. For any
further information, please check out the [project page](https://github.com/web2py/pydal).

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

```python
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.