title: Django Rest Framework Get Additional related fields in one to many relation
body: <p>Here's the problem Products have multiple sizes which is many to many relation </p>
  
  <p><strong>Products</strong></p>
  
  <pre><code>|-------|---------|
  |   ID  |  Title  |
  |-------|---------|
  </code></pre>
  
  <p><strong>Sizes</strong></p>
  
  <pre><code>|-------|---------|
  |   ID  |  Title  |
  |-------|---------|
  </code></pre>
  
  <p><strong>Size Through</strong></p>
  
  <pre><code>|------|--------------|-----------|---------|
  |  ID  |  Product ID  |  Size ID  |  Stock  |
  |------|--------------|-----------|---------|
  </code></pre>
  
  <p>I manage to get sizes in my product api using Django Rest Framework but i don't know how to get stock information as well.
  here is the code I've </p>
  
  <pre><code>class SizeSerializer(serializers.ModelSerializer):
      class Meta:
          fields = (
              'id',
              'title',
          )
          model = Size
  
  
  
  class ProductsSerializer(serializers.ModelSerializer):
      sizes = SizeSerializer(many=True)
  
      class Meta:
          fields = (
              'url',
              'id',
              'title',
              "sizes",
          )
          model = Products
  </code></pre>
  
  <p>I would appreciate any help in this problem</p>
  
  <p><strong>Models</strong></p>
  
  <pre><code>class Products(models.Model):
      title = models.CharField(max_length=125)
      sizes = models.ManyToManyField(Size, blank=True, through='SizeThrough')
  
      class Meta:
          verbose_name_plural = 'Products'
          verbose_name = 'Product'
  
      def __str__(self):
          return self.title
  
  class SizeThrough(models.Model):
      size = models.ForeignKey(Size, on_delete=models.CASCADE)
      product = models.ForeignKey(Products, on_delete=models.CASCADE)
      stock = models.BooleanField(null=False, default=1)
  
  class Size(models.Model):
      title = models.CharField(max_length=20)
  
      def __str__(self):
          return self.title
  </code></pre>