DRF, add custom field to ModelSerializer

Question

I have some models in my project and I need a especial response of the API, i'm using Django Rest framework.

class Goal(models.Model):
    name = models.CharField()
    # more fields

class Task(models.Model):
    name = models.CharField()
    goal = models.ForeignKey(Goal)

class UserTask(models.Model):
    goal = models.ForeignKey(Goal)
    user = models.ForeignKey(User)
    # other fields

I have this response:

{
  "name": "One goal",
  "task": [
    {
      "name": "first task"
    },
    {
      "name": "second tas"
    }
  ]
}

But I need this:

{
  "name": "One goal",
  "task": [
    {
      "name": "first task",
      "is_in_usertask": true
    },
    {
      "name": "second tas",
      "is_in_usertask": false
    }
  ]
}

I saw this in DRF docs but I don't know how to filter UserTask by the current user (or other that is given in URL paramenter) and each Goal.

Edit:

# serializers

class TaskSerializer(serializers.ModelSerializer):
    class Meta:
        model = Task


class GoalSerializer(serializers.ModelSerializer):
    # related_name works fine
    tasks = TaskSerializer(many=True)

    class Meta:
        model = Goal

Answer 1

try to use SerializerMethodField field as

class TaskSerializer(serializers.ModelSerializer):
    is_in_usertask = serializers.SerializerMethodField(read_only=True)

    class Meta:
        model = Task
        fields = ('name', 'is_in_usertask')

    def get_is_in_usertask(self, task):
        return UserTask.objects.filter(user=self.context['request'].user, goal=task.goal).exists()