Flask Marshmallow: Serializing Many-to-Many Relations with Extra Field

Are you facing challenges in serializing model objects with a many-to-many relationship and an extra field stored in an association table? Do you want to generate serialized data that includes embedded objects and additional information? In this blog post, we will explore how to tackle this issue using Flask, SQLAlchemy, and Marshmallow.

Understanding the Problem

When dealing with a many-to-many relationship and an extra field, it can be tricky to serialize the data in a desired format. In your case, you want to achieve a serialized data structure that includes the main object, its associated interests, and the extra field values.

The Scenario

Let’s consider a scenario where you have three main models: Owners, Interests, and an association table called OwnerInterests. The OwnerInterests table stores the extra field value for each owner-interest combination. Your goal is to generate a JSON response that includes all the necessary information in the desired format.

Building the Models

To begin, you need to define the models in SQLAlchemy. The Owners model represents the main object, while Interests represents the associated interests. The OwnerInterests table serves as the association table between Owners and Interests, storing the extra field value.

The Code

Here’s an example of how you can define the models:

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class OwnerInterests(db.Model):
    owner_id = db.Column(db.Integer, db.ForeignKey('owners.id'), primary_key=True)
    interest_id = db.Column(db.Integer, db.ForeignKey('interests.id'), primary_key=True)
    active = db.Column(db.Boolean)
    interest = db.relationship('Interests', back_populates='owners')
    owner = db.relationship('Owners', back_populates='interests')


class Owners(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)
    mobile = db.Column(db.String)
    interests = db.relationship('OwnersInterests', back_populates='owner')


class Interests(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String)
    owners = db.relationship('OwnersInterests', back_populates='interest')

By setting up the relationships between the models, you establish the necessary connections for retrieving and serializing the data.

Serializing with Marshmallow

Now that you have your models defined, it’s time to focus on the serialization process. Marshmallow provides a convenient way to define schemas that determine how the data should be serialized.

The Marshmallow Schema

You can create separate schemas for each model and define the desired fields for serialization. Here’s an example of how the schema for Interests could be defined:

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from marshmallow import Schema, fields

class InterestSchema(Schema):
    class Meta:
        fields = ('id', 'name')
        ordered = True

In this schema, we specify that we want to include the ‘id’ and ‘name’ fields in the serialized output.

Handling the Many-to-Many Relationship

To handle the many-to-many relationship, we can create a schema for the association table, OwnerInterests. This schema includes a nested field for the InterestSchema.

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class OwnerInterestSchema(Schema):
    interest = fields.Nested(InterestSchema)

    class Meta:
        fields = ('id', 'interest', 'active')
        ordered = True

By nesting the InterestSchema within the OwnerInterestSchema, we can achieve the desired structure in the serialized data.

Serializing the Owners

Finally, we can create the schema for the Owners model, which includes a nested field for the OwnerInterestSchema.

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class OwnerSchema(Schema):
    interests = fields.Nested(OwnerInterestSchema, many=True)

    class Meta:
        fields = ('id', 'name', 'mobile', 'interests')
        ordered = True

By nesting the OwnerInterestSchema within the OwnerSchema, we can generate the serialized data structure that includes all the necessary information.

Putting It All Together

Now that you have the models and schemas defined, you can easily serialize the data using Marshmallow. Here’s an example of how you can retrieve an Owner object from the database and serialize it:

code






from app.serialisation import OwnerSchema
from app.models import Owner

owner = Owner.query.get(1)
owner_data = OwnerSchema().dump(owner)

print(owner_data)

The output will be a JSON representation of the Owner object, including the associated interests and their respective extra field values.

Conclusion

Serializing many-to-many relations with an extra field can be challenging, but with Flask, SQLAlchemy, and Marshmallow, you have the tools to overcome this obstacle. By defining the appropriate models and schemas, you can generate the desired serialized data structure effortlessly.

In this blog post, we explored the concept of serializing many-to-many relations with extra fields using Flask, SQLAlchemy, and Marshmallow. We learned how to define models, create schemas, and serialize the data accordingly. Armed with this knowledge, you can confidently tackle similar serialization challenges in your Flask applications.