Additional field while serializing django rest framework

I am a newbie to django rest framework and have created a sample Employee model.

My models.py:

class Employees(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)

My serializers.py:

class EmployeeSerializer(serializers.Serializer):
    class Meta:
        model = Employees
        fields = ('first_name','last_name')

This works fine but I want an additional field full_name, which will be first_name + last_name.

How do I define this new field full_name in my serializers.py?

I see two ways here (I prefer the first way since you can reuse it in other parts of the app):

add a calculated property to your model and add it to your serializer by using a readonly field with source=

# models.py
class Employees(models.Model):
    created = models.DateTimeField(auto_now_add=True)
    first_name = models.CharField(max_length=100)
    last_name = models.CharField(max_length=100)

    @property
    def full_name(self):
        return self.first_name + self.last_name

# serializers.py
class EmployeeSerializer(serializers.ModelSerializer):
    full_name = serializers.Field(source='full_name')

    class Meta:
        model = Employees
        fields = ('first_name','last_name', 'full_name')

by using SerializerMethodField (your model unchanged)

class EmployeeSerializer(serializers.ModelSerializer):
    full_name = serializers.SerializerMethodField('get_full_name')

    def get_full_name(self, obj):
        return obj.first_name + obj.last_name

    class Meta:
        model = Employees
        fields = ('first_name','last_name', 'full_name')

Provided that the Employee is a login user, then most of us will use django.auth.User, I will share how Employee can be implemented as another Profile (extension of django User). Also with the addition of full_name.read_only, first_name.write_only, and last_name.write_only

# models.py
class Employee(models.Model):
    """User Profile Model"""
    user = models.OneToOneField('auth.User')

# serializers.py
class EmployeeSerializer(serializers.HyperlinkedModelSerializer):
    username = serializers.CharField(source='user.username')
    email = serializers.EmailField(source='user.email')

    first_name = serializers.CharField(
            source='user.first_name', write_only=True)
    last_name = serializers.CharField(
            source='user.last_name', write_only=True)
    name = serializers.CharField(
            source='user.get_full_name', read_only=True)
    

    class Meta:
        model = Employee
        fields = (
            'url', 'username', 'email',
            'first_name', 'last_name', 'name')
        depth = 1

SerializerMethodField works fine, and we can also store data in serializer object and let method get_field_name use that.

Example:

class MySerializer(serializers.ModelSerializer):
    statistic = serializers.SerializerMethodField()

    def __init__(self, instance=None, data=serializers.empty, statistic=None, **kwargs):
        super(MySerializer, self).__init__(instance=instance, data=data, **kwargs)
        self.statistic = statistic

    def get_statistic(self, obj):
        if self.statistic is None:
            return serializers.empty
        return self.statistic.get(obj.id, {})