Good to see at another lesson! In this unit, we will explore how to handle and validate complex data structures by creating nested schemas. This is an essential skill when dealing with real-world data, which often includes nested relationships such as user profiles with nested address details.

By the end of this lesson, you will be able to:

• Define and integrate nested schemas using Marshmallow.
• Validate nested data structures within a Flask endpoint.

Let's dive in!

Before we get started, let's briefly recap our basic Flask setup. This time we also introduced address information to our mock database to demonstrate handling nested data more effectively:

Python
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1from flask import Flask
2
3# Initialize a Flask app instance
4app = Flask(__name__)
5
6# Mock database as a list of dictionaries
7database = [
8    {"id": 1, "username": "cosmo", "email": "cosmo@example.com", "address": {"street": "123 Cosmic St", "city": "Cosmopolis"}},
9    {"id": 2, "username": "jake", "email": "jake@example.com", "address": {"street": "456 Jake Blvd", "city": "Jaketown"}},
10    {"id": 3, "username": "emma", "email": "emma@example.com", "address": {"street": "789 Emma Rd", "city": "Emmaville"}}
11]

Now, let's build on this by adding functionality to handle nested data structures.

First, we need to understand what nested schemas are. Nested schemas allow us to represent and validate complex data structures, where one schema is nested within another. For example, a user may have an address with its own schema.

We'll start by defining an AddressSchema for handling nested address data:

Python
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1from marshmallow import Schema, fields
2
3# Define a nested schema for address
4class AddressSchema(Schema):
5    street = fields.Str(required=True)
6    city = fields.Str(required=True)

In this example:

• We define the AddressSchema with two required fields: street and city.
• The required=True argument ensures that both fields must be provided and follow their respective data types (string).

Next, we integrate the AddressSchema into the main UserSchema to model complex user data that includes address information:

Python
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1# Define a main schema for user with nested address
2class UserSchema(Schema):
3    id = fields.Int()
4    username = fields.Str(required=True)
5    email = fields.Email(required=True)
6    address = fields.Nested(AddressSchema, required=True)
7
8# Create an instance of the User schema
9user_schema = UserSchema()

In this main schema, we introduce the address field using fields.Nested(). This means that the address field must conform to the structure and validation rules defined in the AddressSchema, and with the required=True argument, it ensures that the address field is mandatory.

No changes are needed to our existing POST endpoint, as the validation of both the main UserSchema and the nested AddressSchema will happen automatically:

Python
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1from flask import request, jsonify
2from marshmallow import ValidationError
3
4# Define a route to handle user creation
5@app.route('/users', methods=['POST'])
6def create_user():
7    try:
8        # Validate the incoming JSON data
9        user_data = user_schema.load(request.get_json())
10    except ValidationError as err:
11        # Return validation errors as JSON response
12        return jsonify(error=err.messages), 400
13
14    # Generate a new ID by finding the maximum existing ID and adding 1
15    new_id = max(user['id'] for user in database) + 1
16    user_data["id"] = new_id
17    # Add the new user to the mock database
18    database.append(user_data)
19    # Return the newly created user data as JSON response
20    return jsonify(user_data), 201

If the incoming request does not meet the validation criteria defined in the UserSchema or the nested AddressSchema, the ValidationError exception will be raised, and the response will detail the specific validation errors.

For example, if the city field in the nested address data is missing, the response will look like this:

JSON
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1{
2    "error": {
3        "address": {
4            "city": [
5                "Missing data for required field."
6            ]
7        }
8    }
9}

This response is returned with a 400 status code, indicating invalid data provided by the client.

In this lesson, we learned how to handle and validate complex data structures using nested schemas in Marshmallow:

• We defined nested schemas to model complex data structures.
• We integrated nested schemas into a main schema.
• We validated and handled nested data within a Flask endpoint.

With these skills, you're now ready to tackle the practice exercises that follow this lesson. These exercises will give you hands-on experience and reinforce your understanding of nested schemas in Marshmallow.

Congratulations on making it this far! Keep practicing, and happy coding!