我在看一个视频，作者说在连接表上有主键是不好的，但没有解释原因.
示例中的连接表在Rails迁移中定义了两列，作者为每列添加了索引，但没有添加主键.
在这个例子中，为什么主键不好？
create_table :categories_posts, :id => false do |t|
t.column :category_id, :integer, :null => false
t.column :post_id, :integer, :null => false
end
add_index :categories_posts, :category_id
add_index :categories_posts, :post_id
EDIT: As I mentioned to Cletus, I can understand the potential usefulness of an auto number field as a primary key even for a join table. However in the example I listed above, the author explicitly avoids creating an auto number field with the syntax ":id => false" in the "create table" statement. Normally Rails would automatically add an auto-number id field to a table created in a migration like this and this would become the primary key. But for this join table, the author specifically prevented it. I wasn't sure why he decided to follow this approach.