def user(model):
    if not model.is_authenticated():
        return {}

    return {
        'id': model.id,
        'display_name': model.display_name,
        'first_name': model.first_name,
        'last_name': model.last_name,
    }


def category(model):
    return {
        'id': model.id,
        'name': model.name,
        'parent': model.parent_id,
    }


def conversation(model):
    participants = [_['id'] for _ in model.participants.order_by('id').values('id')]
    newest_message = model.messages.order_by('-created_at').first()
    return {
        'id': model.id,
        'name': model.name,
        'participants': participants,
        'message': conversation_message(newest_message),
    }


def conversation_message(model):
    return {
        'id': model.id,
        'sender': model.sent_by_id,
        'created_at': model.created_at.isoformat(),
        'content': model.content,
    }

def user(model):
    if not model.is_authenticated():
        return {}

    return {
        'id': model.id,
        'display_name': model.display_name,
        'first_name': model.first_name,
        'last_name': model.last_name,
    }


def category(model):
    return {
        'id': model.id,
        'name': model.name,
        'parent': model.parent_id,
    }


def conversation(model):
    participants = [_['id'] for _ in model.participants.order_by('id').values('id')]
    newest_message = model.messages.order_by('-created_at').first()
    return {
        'id': model.id,
        'name': model.name,
        'participants': participants,
        'message': conversation_message(newest_message),
    }


def conversation_message(model):
    if model:
        return {
            'id': model.id,
            'sender': model.sent_by_id,
            'created_at': model.created_at.isoformat(),
            'content': model.content,
        }
    else:
        return None

Allow empty conversations to be serialized
Allow empty conversations to be serialized

A conversation may exist without any content. The serializer then
returns an empty message value.

  def user(model):
      if not model.is_authenticated():
          return {}
  
      return {
          'id': model.id,
          'display_name': model.display_name,
          'first_name': model.first_name,
          'last_name': model.last_name,
      }
  
  
  def category(model):
      return {
          'id': model.id,
          'name': model.name,
          'parent': model.parent_id,
      }
  
  
  def conversation(model):
      participants = [_['id'] for _ in model.participants.order_by('id').values('id')]
      newest_message = model.messages.order_by('-created_at').first()
      return {
          'id': model.id,
          'name': model.name,
          'participants': participants,
          'message': conversation_message(newest_message),
      }
  
  
  def conversation_message(model):
+     if model:
-     return {
+         return {
-         'id': model.id,
+             'id': model.id,
-         'sender': model.sent_by_id,
+             'sender': model.sent_by_id,
-         'created_at': model.created_at.isoformat(),
+             'created_at': model.created_at.isoformat(),
-         'content': model.content,
+             'content': model.content,
-     }
+         }
+     else:
+         return None
  
Allow empty conversations to be serialized
## Code Before:
def user(model):
    if not model.is_authenticated():
        return {}

    return {
        'id': model.id,
        'display_name': model.display_name,
        'first_name': model.first_name,
        'last_name': model.last_name,
    }


def category(model):
    return {
        'id': model.id,
        'name': model.name,
        'parent': model.parent_id,
    }


def conversation(model):
    participants = [_['id'] for _ in model.participants.order_by('id').values('id')]
    newest_message = model.messages.order_by('-created_at').first()
    return {
        'id': model.id,
        'name': model.name,
        'participants': participants,
        'message': conversation_message(newest_message),
    }


def conversation_message(model):
    return {
        'id': model.id,
        'sender': model.sent_by_id,
        'created_at': model.created_at.isoformat(),
        'content': model.content,
    }

## Instruction:
Allow empty conversations to be serialized
## Code After:
def user(model):
    if not model.is_authenticated():
        return {}

    return {
        'id': model.id,
        'display_name': model.display_name,
        'first_name': model.first_name,
        'last_name': model.last_name,
    }


def category(model):
    return {
        'id': model.id,
        'name': model.name,
        'parent': model.parent_id,
    }


def conversation(model):
    participants = [_['id'] for _ in model.participants.order_by('id').values('id')]
    newest_message = model.messages.order_by('-created_at').first()
    return {
        'id': model.id,
        'name': model.name,
        'participants': participants,
        'message': conversation_message(newest_message),
    }


def conversation_message(model):
    if model:
        return {
            'id': model.id,
            'sender': model.sent_by_id,
            'created_at': model.created_at.isoformat(),
            'content': model.content,
        }
    else:
        return None
