Optional file storage for data persistence

[ikondov]

The persistence topic has been already addressed in issues #350 and #406. We have another use case where the tests of the command line interface require saving the database and making it available across several processes. Therefore, some temporary file storage is necessary.

This can be accomplished by a small extension of the classes in store.py. The file store is not intended to keep database state instantly on disk. Rather the database is eventually dumped to a file just before the ServerStore object is destroyed.

It is a work in progress: TODO: a transparent method to pass the option to use a database file with a database file path. This can be accomplished e.g. by using an environment variable.

The persistence is activated either by setting the environment variable MONGOMOCK_SERVERSTORE_FILE to a non-empty JSON file (initialized with '{}'):

export MONGOMOCK_SERVERSTORE_FILE=/full/path/to/mongomock_file.json

or create a custom ServerStore object by passing the filename keyword argument:

from mongomock.store import ServerStore
from mongomock import MongoClient

mongo_client = MongoClient(_store=ServerStore(filename='/full/path/to/mongomock_file.json'))

[ikondov]

@pcorpet: Addressing your suggestion in #350, the Database API is not modified at all. Only the storage classes are extended with serialization methods and the ServerStore class with a file dump method. The methods to activate persistence are tested in the test module (see test__persistence.py). By default it is deactivated.