Add the ability to filter queries before construction of the CTE

[rhomboss]

• Update query.py -Add pre_filter and pre_exclude methods to TreeQuerySet. -Make query methods call _setup_query to deal with sibling order and pre_filter persistence issues.

• Update compiler.py -Replace the get_sibling_order_params with get_rank_table_params to support early tree filtering. -Change sibling_order and pre_filter from class variables to variables that have to be initiated by _setup_query so that they don't persist between user queries. -Handle pre_filter params by passing them to the django backend.

• Update test_queries.py -Added tests for the pre_filter and pre_exclude methods.

[rhomboss]

This addresses issue #50 and also addresses a possible bug where sibling ordering was persisting between queries even when no explicit ordering was given because it was being stored as a class variable.

It should be noted that filtering prior to the construction of the CTE means that filtered status is inherited i.e. if a parent is excluded so too will all of its descendants. This contrasts with a normal filter which when applied after the CTE is built will retain the descendants of an excluded parent if those descendants meet the filtering criteria.

[rhomboss]

If you think tree_filter is a better name I can change it. When I was writing this over the weekend I really only thought of the filter as something that applied to the tree earlier than the normal filter.

I had completely forgotten about Q objects. Using them exclusively won't really simplify the code any since all that really happens is the arguments of pre_filter gets passed directly into django's normal queryset .filter. That being said I did forget to add support for Q objects which is a pretty easy fix. I just have to make sure that pre_filter passes both *args and **kwargs instead of just **kwargs.

[rhomboss]

I had an idea for how to simplify this feature by making the Django ORM generate the entire __rank_table by itself. I'm still investigating but it looks promising and should simplify any future API extensions that involve the __rank_table so don't commit any of my changes just yet!

[rhomboss]

I've simplified the code by adding the instance variable called rank_table_query. This is a default Django queryset that can be used to generate the CTE's __rank_table sql. Now, the tree_filter and tree_exclude methods simply apply Django's normal queryset methods to the rank_table_query. And, in the future if there is need to allow users to further modify the __rank_table beyond basic filtering you only have to expose the appropriate Django queryset function with the API.

[matthiask]

Thank you!