sqlalchemy sql composition

I am working on a codebase that uses sqlalchemy extensively and I am new to it. I am having trouble composing a sqlalchemy expression for a query that I want to write.

We have the following 3 tables:

1. Product - (product_id, product_description, status, available)
2. Category - (category_id, category_description)
3. ProductCategoryLink - (product_id, category_id) [For many-to-many relationship]

I am running a query that gives me a product count per category. Some products are not assigned to any category and I want those products as well (category will be null in that case). I came up with the following query

select c.category_name, count(p.product_id)
from Product as p
left join ProductCategoryLink as pc
  on p.product_id = pc.product_id
left join Category as c      
  on c.category_id = pc.category_id
where p.store_id = 1111      
and p.status = 'ACTIVE'      
and p.available = 1
group by c.category_name; 

I have the following mappings in my orm file

class Product(Base):
    __tablename__ = 'Product'

    product_id          = Column(Integer, primary_key=True, name='product_id')
    product_description = Column(UnicodeText, name='description')
    available           = Column(Boolean, name='available')
    status              = Column(Unicode(length=255), name='status')

metadata = Base.metadata
# association table between product and category
product_category_link = Table('ProductCategoryLink', metadata,
        Column('product_id', Integer, ForeignKey('Product.product_id')),
        Column('category_id', Integer, ForeignKey('Category.category_id'))
)

class Category(Base):
    __tablename__ = 'Category'

    category_id                 = Column(Integer, primary_key=True, name='category_id')
    category_name               = Column(Unicode(length=255), name='category_name')
    products                    = relation('Product', secondary=product_category_link, backref='categories')

I came up with the following ORM expression

    query = session.query(Category.category_name, func.count(Product.product_id)).join(product_category_link).\
            join(Category).filter(
                and_(Product.store_id == self._store_id,
                    and_(Product.status == 'ACTIVE', Product.available == 1))).\
            group_by(Category.category_name).all()

The sql query that the above expression creates is not what I want.

sqlalchemy.exc.OperationalError: (OperationalError) (1066, "Not unique table/alias: 'Category'") 'SELECT `Category`.category_name AS `Category_category_name`, count(`Product`.product_id) AS count_1 \nFROM `Product`, `Category` INNER JOIN `ProductCategoryLink` ON `Category`.category_id = `ProductCategoryLink`.category_id INNER JOIN `Category` ON `Category`.category_id = `ProductCategoryLink`.category_id \nWHERE `Product`.store_id = %s AND `Product`.status = %s AND `Product`.available = %s GROUP BY `Category`.category_name' (1, 'ACTIVE', 1)

What am I doing wrong here?