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Using Djongo Array Reference Field |
/using-django-with-mongodb-array-reference-field/ |
The ArrayReferenceField
is one of the most powerful features of Djongo. The ArrayModelField
stores the embedded models within a MongoDB array as embedded documents for each entry. If entries contain duplicate embedded documents, using the ArrayModelField
would require unnecessary disk space. The ManyToManyField
on the other hand has a separate table for all the entries. In addition, it also creates an intermediate "through/join" table which records all the mappings.
The ArrayReferenceField
is a bargain between the ArrayModelField
and ManyToManyField
. A separate collection is used for storing all entries (instead of embedding it as an array). This means there is no data duplication. However, the intermediate "through/join" mapping table is completely skipped! This is achieved by storing only a reference to the entries in the embedded array.
While the ManyToManyField
required two queries to fetch data, the ArrayReferenceField
requires just one query and is much faster. If you have used the ManyToManyField
, then you know how to use the ArrayReferenceField
. In fact it implements the exact same API as the ManyToManyField
. You can replace all existing ManyToManyField
with ArrayReferenceField
and everything will continue to work as is.
In the example the Entry
Model can be rewritten as follows:
class Author(models.Model):
name = models.CharField(max_length=200)
email = models.EmailField()
def __str__(self):
return self.name
class Entry(models.Model):
blog = models.EmbeddedModelField(
model_container=Blog,
model_form_class=BlogForm
)
meta_data = models.EmbeddedModelField(
model_container=MetaData,
model_form_class=MetaDataForm
)
headline = models.CharField(max_length=255)
body_text = models.TextField()
authors = models.ArrayReferenceField(
to=Author,
on_delete=models.CASCADE,
)
n_comments = models.IntegerField()
def __str__(self):
return self.headline
Notice how the Author
model is no longer set as abstract
. This means a separate author
collection will be created in the DB. Simply set the authors
to a list containing several author instances. When the entry gets saved, only a reference to the primary_key of the author model is saved in the array. Upon retrieving an entry from the DB the corresponding authors are automatically looked up and the author list is populated.
The ArrayReferenceField
behaves exactly like the ManyToManyField
. However, underneath only references to the entries are being stored in the array.