5.2 Declaring and using a nested EditingContext

5.2 Declaring and using a nested EditingContext

Creating a nested EditingContext

You create a nested EditingContext by simply supplying the parent EditingContext to the initializer:

>>> from Modeling.EditingContext import EditingContext
>>> parent_ec=EditingContext()
>>> child_ec=EditingContext(parent_ec)

Fetching objects

You fetch objects exactly the same way you do with a standard EditingContext, as described in section 4.5, ''Fetching objects''.

However, the result set you'll get will probably be different than the one you would get by asking a standard EditingContext, because:

every new object inserted in the parent EditingContext matching the fetch specification will show up in the result,
you won't see any of the objects that are marked as deleted in the parent EditingContext,
the states of the objects in the result set will reflect any changes made to them in the parent object store, even the uncommitted ones.

Current operations

All currently supported operations on an EditingContext, such as inserting or deleting an object, making changes and validating them, are available in a child EditingContext without any modifications to your code.

Discarding or saving changes

You save the changes made in a child EditingContext to its parent just by calling saveChanges() on it. Remember: a nested EditingContext saves its changes to its parent, NOT to the database. If you want to make them persistent, you will need the two following steps:

>>> child_ec.saveChanges()  # save changes to the parent_ec
>>> parent_ec.saveChanges() # save the changes to the database

Note: That's the reason why the fourth of ACID properties, Durability is not supported: since the changes are saved in the parent EditingContext and not to the database itself, they will be obviously lost if the application terminates abruptly.

Discarding the changes made in a child EditingContext is as simple as forgetting the child (e.g. dy deleting it). Remember that a parent EditingContext will never see the changes made in its children until saveChanges() is explicitly called on them.

Grand-children of an EditingContext

Is it possible to declare a child of an already nested EditingContext? Yes! Every EditingContext can have children, irrespective of whether it is already nested; the depth of a hierarchy defined by parent/children EditingContext is indeed unlimited.