MongoListProxy

Data Structure

A MongoListProxy manages a list of subdocuments as shown in the following structure. See Proxy Overview for a list of all supported data structures.

{                              # parent document
    '_id' : ...,
    'tickets' : [              # container is a list
        {
            '_sdkey' : '1',    # subdocument with key '1'
            ...
        },
        {
            '_sdkey' : '2',    # subdocument with key '2'
            ...
        },
        {
            '_sdkey' : '3',    # subdocument with key '3'
            ...
        },
    ],
}

This data structure can be accessed as:

class Tickets( mongo_objects.MongoListProxy ):
    container_name = 'tickets'

class Events( mongo_objects.MongoUserDict ):
    collection_name = 'events'
    database = ... pymongo database connection object ...

Proxy Keys

Assigning Keys on Create

Python lists don’t natively use keys to access content, so the subdocument proxy key is added to each subdocument dictionary under the key _sdkey.

We recommend using the built-in system of auto-assigning unique integers as proxy keys to avoid changing keys and invalidating existing documents or URLs.

Locating Subdocuments

It is too slow to iterate through the list of subdocuments on every access to find the correct key. MongoListProxy makes the following look-up optimization.

Each proxy object stores both the key and a sequence number representing the list index that held that key during the most recent data access.

  1. When accessing the proxy object, the subdocument at the currently saved sequence number is examined. If the key matches the key for the proxy object, that subdocument is immediately returned.

  2. Otherwise, the list is rescanned from the front until a subdocument is located with the correct proxy key. Once the key is located, the sequence number is stored as a hint for next time and the subdocument at the sequence location is returned.

Initializing Objects

__init__() accepts three arguments, parent, key and seq. parent is required. At least one of key and seq must also be provided.

# If only key is provided, the key is saved in the object. No validation is

performed. The first time the proxy is accessed, the list will be scanned for the correct subdocument.

# If only seq is provided, the key for the subdocument currently at that location

in the list is saved as the proxy key. The seq value is saved as a hint for the next access. If the list changes, the proxy key will be located again and the sequence number updated.

# If both key and seq are provided, the key is saved in the proxy object but

not validated. The sequence number is saved as a hint for the next access and will be validated at that time.

Class Reference

class mongo_objects.MongoListProxy(parent, key=None, seq=None)

Implement proxy object using a list as a container

classmethod get_proxies(parent)

Return all proxy objects in the container_name list container.

Parameters:

parent (MongoUserDict or proxy object) – parent document or subdocument

Returns:

list of proxy objects

Return type:

MongoListProxy subclasses

classmethod get_proxy(parent, key=None, seq=None)

Return a single proxy object passing the arguments through to calling __init__()

At least one of key and seq must be provided. See __init__() method for how these arguments are handled.

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • key (str) – unique key identifying an existing member of the dictionary container

Returns:

proxy object

Return type:

MongoListProxy subclass

__init__(parent, key=None, seq=None)

Instantiate a new proxy object from a list container.

At least one of key and seq must be provided.

  • If only key is provided, the key is saved in the object. No validation is performed. The first time the proxy is accessed, the list will be scanned for the correct subdocument.

  • If only seq is provided, the key for the subdocument currently at that location in the list is saved as the proxy key. The seq value is saved as a hint for the next access. If the list changes, the proxy key will be located again and the sequence number updated.

  • If both key and seq are provided, the key is saved in the proxy object but not validated. The sequence number is saved as a hint for the next access and will be validated at that time.

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • key (int, optional) – unique key identifying an existing member of the list container

  • seq – index into the list container

Raises:

MongoObjectsNonexistentKey – if key does not already exist in the list container

container_name

Users must override this to provide the name of the dictionary or list container

classmethod create(parent, subdoc=None, autosave=True)

Add a new subdocument to the list container. Auto-assign the ID and return the new proxy object.

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • subdoc (dict) – new subdocument to be added

  • autosave (bool) – should the parent document be saved after this new content is added

Returns:

a proxy object to the new data

Return type:

MongoDictProxy subclass

classmethod create_key(parent, subdoc)

Create a unique key value for this subdocument. The default implementation requests a hex string for the next unique integer as saved in the MongoUserDict parent object.

Users may override this using data from the subdoc or other methods to generate a unique key. It is recommended that the key not be changed once the object is created as it will invalidate any existing proxies and any subdocument_id strings.

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • subdoc (dict) – the subdocument for which a new key is being created. The default implementation ignores this parameter but users may find it useful when overriding this method.

Returns:

a unique string suitable for use as a MongoDB dictionary key

data()

Convenience method to behave similar to UserDict.

Returns:

The subdocument dictionary proxied from the parent document

Return type:

dict

delete(autosave=True)

Delete the subdocument from the container list. Remove the key and sequence so the proxy can’t be referenced again. By default save the parent document.

Once a proxy object is deleted the underlying dictionary no longer exists so all values disappear immediately.

Parameters:

autosave (bool) – should the parent document be saved after the proxy content is deleted

classmethod exists(parent, key)

Return True if the key already exists in the list container

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • key (str) – key being queried

get(key, default=None)

Get a value from the subdocument dictionary

id()

Return the full proxy ID including the parent document ID. This is really just a wrapper around proxy_id() but keeps the parallel with MongoUserDict.

items()

Return the items in the subdocument dictionary

keys()

Return the keys in the subdocument dictionary

proxy_id(*args, include_parent_doc_id=False)

Assemble a list of proxy IDs into a single string. Each proxy passes the request up to it’s parent adding each ID (key) along the way.

Parameters:

include_parent_doc_id (bool) – whether to include the parent document ID in the resulting ID string

Returns:

One or more proxy IDs separated by subdoc_key_sep

Return type:

str

save()

Saving the subdocument means saving the parent object, so we simply pass the save request up the line.

setdefault(key, default=None)

Set a default value in the subdocument dictionary

update(values)

Update the subdocument dictionary with new values

values()

Return the values in the subdocument dictionary

Polymorphic Class Reference

Polymorphic proxies are supported by PolymorphicMongoListProxy. All the attributes and methods of MongoListProxy are supported with the following overrides.

class mongo_objects.PolymorphicMongoListProxy(parent, key=None, seq=None)

Polymorphic version of MongoListProxy

classmethod get_proxies(parent)

Return all proxy objects in the container_name list container with the correct polymorphic subclasses.

Parameters:

parent (MongoUserDict or proxy object) – parent document or subdocument

Returns:

list of proxy objects

Return type:

MongoListProxy subclasses

classmethod get_proxy(parent, key=None, seq=None)

Return a single list proxy object. Determine the correct subclass type and pass the arguments to __init__()

At least one of key and seq must be provided. See __init__() method for how these arguments are handled.

Parameters:
  • parent (MongoUserDict or proxy object) – parent document or subdocument

  • key (str, optional) – the key identifying this subdocument

  • seq (int, optional) – the current index in the list container of the subdocument

Returns:

proxy object

Return type:

PolymorphicMongoListProxy subclass

Raises:

MongoObjectsPolymorphicMismatch – if the resulting object is not of type cls or a subclass of cls