API Python/RQL

~~~~~~~~~~~~~~

The Python API developped to interface with RQL is inspired from the standard db-api,

with a Connection object having the methods cursor, rollback and commit essentially.

The most important method is the `execute` method of a cursor :

`execute(rqlstring, args=None, cachekey=None, build_descr=True)`

:rqlstring: the RQL query to execute (unicode)

:args: if the query contains substitutions, a dictionary containing the values to use

:cachekey:

   an implementation detail of the RQL cache implies that if a substitution

   is used to introduce an eid *susceptible to raise the ambiguities in the query

   type resolution*, then we have to specify the corresponding key in the dictionary

   through this argument

The `Connection` object owns the methods `commit` and `rollback`. You *should

never need to use them* during the development of the web interface based on

the *CubicWeb* framework as it determines the end of the transaction depending

on the query execution success.

.. note::

  While executing update queries (SET, INSERT, DELETE), if a query generates

  an error related to security, a rollback is automatically done on the current

  transaction.

Executing RQL queries from a view or a hook

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

When you're within code of the web interface, the db-api like connexion is

handled by the request object. You should not have to access it directly, but

use the `execute` method directly available on the request, eg:

   rset = self._cw.execute(rqlstring, kwargs)

Similarly, on the server side (eg in hooks), there is no db-api connexion (since

you're directly inside the data-server), so you'll have to use the execute method

of the session object.

Important note about proper usage of .execute

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's say you want to get T which is in configuration C, this translates to:

.. sourcecode:: python

   self._cw.execute('Any T WHERE T in_conf C, C eid %s' % entity.eid)

But it can also be written in a syntax that will benefit from the use

of a cache on the RQL server side:

.. sourcecode:: python

   self._cw.execute('Any T WHERE T in_conf C, C eid %(x)s', {'x': entity.eid}, 'x')

Beside proper usage of the `args` argument, notice the latest argument: this is what's called

the cache key. The cache key should be either a string or a tuple containing the names of keys

in args which are referencing eids. *YOU MUST SET THIS PROPERLY* if you don't want weird result

on queries which have ambigous solutions deambiguified by specifing an eid. So the good habit is:

*always put in the cache key all eid keys*.

The syntax tree is build once for the "generic" RQL and can be re-used

with a number of different eid.

Alternativelly, some of the common data related to an entity can be obtained from

the top-level `entity.related()` method (which is used under the hood by the orm

when you use attribute access notation on an entity to get a relation. The above

would then be translated to:

.. sourcecode:: python

   entity.related('in_conf', 'object')

The `related()` method, as more generally others orm methods, makes extensive use

of the cache mechanisms so you don't have to worry about them. Additionnaly this

use will get you commonly used attributes that you will be able to use in your

view generation without having to ask the data backend.