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SQLAlchemy Examples

发表于: 2007-06-02   作者:huangyiiiiii   来源:转载   浏览:
摘要: 看 SQLAlchemy 自带的 zblog 的例子,可以看到 SQLAlchemy 一些非常有用的特性。 文章相关评论数统计 比如显示文章列表的同时我们希望获得相关文章的评论数,如果是用 django 那就只能放弃 ORM 的好处自己去执行 sql 语句了,否则就只会导致 n+1 条 SQL 语句的执行。在 SQLAlchemy 中你可以把任意的 select 语句映射到一个 cla
看 SQLAlchemy 自带的 zblog 的例子,可以看到 SQLAlchemy 一些非常有用的特性。

文章相关评论数统计

比如显示文章列表的同时我们希望获得相关文章的评论数,如果是用 django 那就只能放弃 ORM 的好处自己去执行 sql 语句了,否则就只会导致 n+1 条 SQL 语句的执行。
在 SQLAlchemy 中你可以把任意的 select 语句映射到一个 class ,这样就可以用一条 SQL 语句搞定,还能获得 ORM 的好处,下面是原封不动拷过来的代码(只调整了下格式):

   # Post mapper, these are posts within a blog.
# since we want the count of comments for each post,
# create a select that will get the posts
# and count the comments in one query.
posts_with_ccount = select(
[c for c in tables.posts.c if c.key != 'body'] + [
func.count(tables.comments.c.comment_id).label('comment_count')
],
from_obj = [
outerjoin(tables.posts, tables.comments)
],
group_by=[
c for c in tables.posts.c if c.key != 'body'
]
) .alias('postswcount')

# then create a Post mapper on that query.
# we have the body as "deferred" so that it loads only when needed,
# the user as a Lazy load, since the lazy load will run only once per user and
# its usually only one user's posts is needed per page,
# the owning blog is a lazy load since its also probably loaded into the identity map
# already, and topics is an eager load since that query has to be done per post in any
# case.
mapper(Post, posts_with_ccount, properties={
'id':posts_with_ccount.c.post_id,
'body':deferred(tables.posts.c.body),
'user':relation(user.User, lazy=True,
backref=backref('posts', cascade="all, delete-orphan")),
'blog':relation(Blog, lazy=True,
backref=backref('posts', cascade="all, delete-orphan")),
'topics':relation(TopicAssociation, lazy=False, private=True,
association=Topic, backref='post')
}, order_by=[desc(posts_with_ccount.c.datetime)])

树形评论
映射如下:
   # comment mapper.  This mapper is handling a hierarchical relationship on itself,
# and contains
# a lazy reference both to its parent comment and its list of child comments.
mapper(Comment, tables.comments, properties={
'id':tables.comments.c.comment_id,
'post':relation(Post, lazy=True,
backref=backref('comments', cascade="all, delete-orphan")),
'user':relation(user.User, lazy=False,
backref=backref('comments', cascade="all, delete-orphan")),
'parent':relation(Comment,
primaryjoin=tables.comments.c.parent_comment_id==tables.comments.c.comment_id,
foreignkey=tables.comments.c.comment_id, lazy=True, uselist=False),
'replies':relation(Comment,
primaryjoin=tables.comments.c.parent_comment_id==tables.comments.c.comment_id,
lazy=True, uselist=True, cascade="all"),
})

很多时候我们需要一次性获取对应一个文章的所有评论,可以用一条 select 先把数据取出,然后手动建立树形结构:
# we define one special find-by for the comments of a post, which is going to make its own
# "noload" mapper and organize the comments into their correct hierarchy in one pass. hierarchical
# data normally needs to be loaded by separate queries for each set of children, unless you
# use a proprietary extension like CONNECT BY.
def find_by_post(post):
"""returns a hierarchical collection of comments based on a given criterion.
uses a mapper that does not lazy load replies or parents, and instead
organizes comments into a hierarchical tree when the result is produced.
"""
q = session().query(Comment).options(noload('replies'), noload('parent'))
comments = q.select_by(post_id=post.id)
result = []
d = {}
for c in comments:
d[c.id] = c
if c.parent_comment_id is None:
result.append(c)
c.parent=None
else:
parent = d[c.parent_comment_id]
parent.replies.append(c)
c.parent = parent
return result

Comment.find_by_post = staticmethod(find_by_post)

SQLAlchemy Examples

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