Mongoengine is an Object Document Mapper (ODM) tool for working with MongoDB in Python. It provides a high-level, object-oriented interface for interacting with the database.

One of the useful features of Mongoengine is the ability to set default values for fields in your document models. This allows you to define a default value that will be used unless a different value is explicitly provided.

Defining a Default Value

To define a default value for a field in Mongoengine, you can use the default attribute when defining the field in your document model. For example, let’s say we have a User model with a name field, and we want the default value of name to be “Unknown” if no value is provided:

from mongoengine import Document, StringField

class User(Document):
    name = StringField(default="Unknown")

In this example, if you create a new User object without providing a value for name, it will be set to “Unknown” by default.

Overriding the Default Value

If you want to provide a different value for a field that has a default value, you can simply assign a new value to the field when creating the document object. For example:

new_user = User(name="John")

In this case, the name field for the new_user object will be set to “John” instead of the default value “Unknown”.

Dynamic Default Values

You can also set dynamic default values for fields in Mongoengine by using callable objects. Callable objects are objects that can be called as functions (using the () operator).

For example, let’s say we want to set the default value of a created_at field to the current date and time. We can achieve this by using the datetime.now function as the default value:

from mongoengine import Document, DateTimeField
from datetime import datetime

class Post(Document):
    created_at = DateTimeField(default=datetime.now)

In this example, the created_at field will be set to the current date and time whenever a new Post object is created.

Conclusion

Setting default values for fields in your Mongoengine document models can be helpful in ensuring that your data has consistent initial values. By using the default attribute, you can define default values that are used unless a different value is explicitly provided.

In addition, you can also use callable objects to set dynamic default values based on the current state of the system.

Mongoengine’s default value feature simplifies the process of working with MongoDB in Python and provides greater flexibility in managing your data.