.. _constraints:

============
Constraints
============

Constraints are a mean to limit browsing of data to a limited set. It's a
different concept from filtering, as this enforces a limitation while
filtering allows the user to change the values at will.

.. _django-syntax:

Django-like syntax
==================

A constraint can be as simple as::

  t = SqlTable('movie', dbproxy=db)
  t.add_constraints(actors__country='ITA', genres__name='drama',
                    year__gte=1950, year__lte=1970)

And browsing of movies will be constrained into dramas with at least one
Italian actor, produced between 1950 and 1970. The double underscore '__'
will allow spanning from one table to a related one (provided a relation was
established at sqlalchemy level) and from attribute name to operator to be
used.  

.. note::

  This syntax is inspired to django_'s ORM syntax.
  The same syntax can be used to add filters to the sqlwidget, to the :ref:`completion
  <filter_completion>` and when ``.reload()`` method is used.

.. _django: http://www.djangoproject.com

query
=======

limitation acts on ``self.query`` directly. Self query can be modified in
any way accepted by ``sqlalchemy`` syntax 

operators
---------

these operators are recognized::

 OPERATORS = {
     'notlike'    : 'not like',
     'like'       : 'like',
     'ilike'      : 'ilike',
     'notilike'   : 'not ilike',
     'iregexp'    : '~*',
     'regexp'     : '~',
     'notiregexp' : '!~*',
     'notregexp'  : '!~',
     'lt'         : '<',
     'gt'         :  '>',
     'lte'        : '<=',
     'gte'        : '>=',
     'eq'         : '=',
     'neq'        : '!=',
     'equal'      : '=',
     'in'         : 'IN',
     'notin'      : 'NOT IN',
     'null'       : 'IS NULL',
     'notnull'    : 'IS NOT NULL',
     }

and ``icontains`` that at the moment map to regexp operators if they exist
or like with % on both sides.

conjunctions
------------

In the example all conditions have been ``AND`` ed. It's possible to use
``OR`` operator adding ``or_=True``  argument.

django2sqlalchemy
-----------------

You can get a sqlalchemy ``ClauseList`` object starting from a django-like
expression using django2sqlalchemy function::

  from sqlkit.db.django_syntax import django2sqlalchemy
  clause_list, join_paths = django2sqlalchemy(mapper, *args)

where ``args`` can be or_=boolean or any django-like query.
Django2sqlalchemy returns a tuple: (clause_list, join_path):

  :clause_list: 
     the list of column expression connected with ``AND`` or
     ``OR`` according to ``or_`` argument
  :join_path:
     the list of path needed to add to query via ``query.join()`` to
     have all the fields the query needs to apply the filters in clause_list 

     A possibility is to add::
     
        for path in join_path:
	   query.join(path).reset_joinpoint()


Filter simplified syntax
-------------------------

.. automodule:: sqlkit.db.django_syntax

.. autofunction:: django2sqlalchemy


Native sqlalchemy constraint
============================

The django syntax is in no way the only possibility. If that is not
sufficient to express the constraints you need you can just use any filter
on the query directly::

   tk_tbl = ticket.Ticket.__table__
   my_id = self.db.get_session().query(ticket.User).filter_by(username=setup.USERNAME).one().id

   t = sk.SqlTable(ticket.Ticket, field_list=field_list, order_by='priority', **self.meta)
   t.add_filter(status=1)
   t.query = t.query.filter(or_(tk_tbl.c['assigned_by_id'] == my_id ,
                                tk_tbl.c['assigned_to_id'] == my_id , ))


this example shows how to force a constraint on ticket requiring that the
ticket be assigned *by* or *to* USERNAME. Any further filter applied
interactively will be applied *on top* of this constraint. This is
equivalent to writing::

   t.add_constraint(assigned_by_id=my_id, assigned_to_id=my_id, OR=True)

At the moment of this writing it's not possible to write::

   t.add_constraint(assigned_by_id__username=USERNAME)

as the foreign key relation is not followed in this context.

aliased constraints
--------------------

Sometimes you need to have aliased constraints in order to ha sqlalchemy
build an aliased join to related classes on which you may want to set 
constraints. This may be necessary if you want to use constraints along with
order_by and possibly filters on foreign keys.

example
+++++++

Suppose you have an 'address' table with a ``user_id`` field that is a
ForeignKey to a ``user.id`` column. Suppose you want to open a SqlTable on
table addresses, ordered on ``user_id`` (well... surely you don't want to
order by the ``id`` value, you probably have a ``first_name`` field that is
much more appropriate for sorting).  

Now suppose you also have an ``active`` field on the user_table. You can
achieve this (simple) in the following way::

  t = SqlTable('address', order_by='user_id__first_name', ...)
  t.add_constraint(user_id__active=True, aliased=True)

the order_by argument implicitly have built a join with the user table, the
same stands for the constraint ``user_id__active``. Sqlalchemy would have
complained that the table 'user' was already present, so you need to alias
it. I'll probably make this the default in a future version.


