Django retrieve data from multiple tables without foreign key relationship

Question

I have the following three Models

class Post(models.Model):
    category_type = models.CharField(max_length=10)
    participant_id = models.IntegerField()
    title = models.CharField(max_length=200)

class Book(models.Model):
    title = models.CharField(max_length=50)
    author = models.CharField(max_length=500)
    price = models.IntegerField(default=0)

class Instrument(models.Model):
    title = models.CharField(max_length=50)
    price = models.IntegerField(default=0)

Here category_type in Post model can be "book" or "instrument" and these categories can increase in the future.

The participant_id column in Post table references the primary key of either Book table or Instrument table based on the category_type.

Now I want to retrieve all the posts and associated data from database.

In MySql, I can do it like this

select post.title,book.title from post,book where ((post.category_type="book" and post.participant_id=book.id))
union
select post.title,instrument.title from post,instrument where ((post.category_type="instrument" and post.participant_id=instrument.id))

But I am unable to do it using django as I cannot have foreign key relationship.

Any help is highly appreciated.

Update: I tried with Django's generic relations by modifying the Post table as below

class Post(models.Model):
    category_type = models.CharField(max_length=10)
    participant_id = models.PositiveIntegerField()
    content_type = models.ForeignKey(ContentType)
    participant_object = GenericForeignKey('content_type', 'participant_id')    
    title = models.CharField(max_length=200)

And then I issued the following query

Post.objects.filter(content_type=ContentType.objects.get_for_model(Book))

But this gives me error

FieldError: Cannot resolve keyword 'content_type' into field. Choices are: category_type, id,  participant_id, title

How can I retrieve the data if I cannot specify it in the condition.

Answer 1

I used Django's generic relations as suggested by Timmy. Following is the modified model for Post

class Post(models.Model):
    category_type = models.CharField(max_length=10)
    participant_id = models.PositiveIntegerField()
    content_type = models.ForeignKey(ContentType)
    participant_object = GenericForeignKey('content_type', 'participant_id')    
    title = models.CharField(max_length=200)

And then I issued the following query

Post.objects.filter(content_type=ContentType.objects.get_for_model(Book))

This gave me the correct results Don't forget to rebuild your DB after modifying the model otherwise you will get error.