Add auto_indexes feature to LazyInstance

[lafrech]

Closes #81.

What do you think about this ?

It works only on documents that are already registered when calling instance.init().

TODO:

• Add tests for drivers other than PyMongo (currently only PyMongo is tested)

• Document the feature

• Also ensure indexes on documents registered after instance.init()?

[lafrech]

Obviously, it works only on documents that are already registered when calling instance.init().

I think this could be addressed in Instance._register_doc.

@touilleMan , waiting for your feedback before going any further. This is not critical to us.

[coveralls]

Coverage increased (+0.07%) to 95.665% when pulling 7c50390 on Nobatek:dev_auto_indexes into 65c8e01 on Scille:master.

[touilleMan]

I think this feature is pretty interesting given calling ensure_index by hand is cumbersome and error prone ;-)

Few remarks:

1. With not implementing this for Instance as well as LazyLoaderInstance ?

2. The trouble with implementing this at the generic instance level is ensure_indexes won't do the same thing depending of the framework used: the indexes will be up and ready for pymongo were for asyncio and twisted only coroutine has been registered (which will be executed in the future... maybe... or can raise a exception which won't be catched...).
   So I would have this feature in the builder given each framework has it own implementation:

class Instance:
    ...
    def register(self, template):
        # overload the default register to add a synchronous creation of indexes if needed
        implementation = super().register(template)
        if self.auto_indexes:
            self.builder.ensure_indexes(implementation)

class LazyLoaderInstance:
    ...
    def init(self, db):
        ...
        # Now we can finish initialization of all the documents
        for doc in self._doc_lookup.values():
            self.builder.ensure_indexes(doc)

class PyMongoBuilder(BaseBuilder):

    BASE_DOCUMENT_CLS = PyMongoDocument

    ...

    def ensure_indexes(self, implementation):
        # For async framework, we create a reactor just for this coroutine so 
        # we end up with something synchronous anyway
        implementation.ensure_indexes()    

3. I don't like putting the auto_indexes option into the document's meta fields: I feel like document declaration should be independent of it execution as much as possible (we are defining a Template after all !). Using auto_indexes or not depend of which instance is used and not on what is the document made of.
   I think it would be better to add an option to the instance.register instead:

instance = Instance(db, auto_indexes=False)

@instance.register(force_auto_indexes_value=True)
class MyDoc(Document):
    ...

[lafrech]

Thank you for your feedback.

1. Why not implement this for Instance as well as LazyLoaderInstance ?

I don't remember why I did not do it for Instance. I think I had a reason, probably a bad one...

2. & 3.

Good points.

register(force_auto_indexes_value=True) was my first intent in #81, but then I changed my mind. I shouldn't have...

[touilleMan]

I'm closing this because pymongo's ensure_index has been deprecated.

This makes creating automatically indexes much more complicated (plenty of complex concurrency bugs there, that's the reason pymongo dropped the feature in the first place).

For instance, this "don't bother user with this" approach created serious perf issues with mongoengine that are now complicated to fix.