"""
QueryChain is a wrapper for sequence of queries.
Features:
* Easy iteration for sequence of queries
* Limit, offset and count which are applied to all queries in the chain
* Smart __getitem__ support
Initialization
^^^^^^^^^^^^^^
QueryChain takes iterable of queries as first argument. Additionally limit and
offset parameters can be given
::
chain = QueryChain([session.query(User), session.query(Article)])
chain = QueryChain(
[session.query(User), session.query(Article)],
limit=4
)
Simple iteration
^^^^^^^^^^^^^^^^
::
chain = QueryChain([session.query(User), session.query(Article)])
for obj in chain:
print obj
Limit and offset
^^^^^^^^^^^^^^^^
Lets say you have 5 blog posts, 5 articles and 5 news items in your
database.
::
chain = QueryChain(
[
session.query(BlogPost),
session.query(Article),
session.query(NewsItem)
],
limit=5
)
list(chain) # all blog posts but not articles and news items
chain = chain.offset(4)
list(chain) # last blog post, and first four articles
Just like with original query object the limit and offset can be chained to
return a new QueryChain.
::
chain = chain.limit(5).offset(7)
Chain slicing
^^^^^^^^^^^^^
::
chain = QueryChain(
[
session.query(BlogPost),
session.query(Article),
session.query(NewsItem)
]
)
chain[3:6] # New QueryChain with offset=3 and limit=6
Count
^^^^^
Let's assume that there are five blog posts, five articles and five news
items in the database, and you have the following query chain::
chain = QueryChain(
[
session.query(BlogPost),
session.query(Article),
session.query(NewsItem)
]
)
You can then get the total number rows returned by the query chain
with :meth:`~QueryChain.count`::
>>> chain.count()
15
"""
from copy import copy
[docs]class QueryChain:
"""
QueryChain can be used as a wrapper for sequence of queries.
:param queries: A sequence of SQLAlchemy Query objects
:param limit: Similar to normal query limit this parameter can be used for
limiting the number of results for the whole query chain.
:param offset: Similar to normal query offset this parameter can be used
for offsetting the query chain as a whole.
.. versionadded: 0.26.0
"""
def __init__(self, queries, limit=None, offset=None):
self.queries = queries
self._limit = limit
self._offset = offset
def __iter__(self):
consumed = 0
skipped = 0
for query in self.queries:
query_copy = copy(query)
if self._limit:
query = query.limit(self._limit - consumed)
if self._offset:
query = query.offset(self._offset - skipped)
obj_count = 0
for obj in query:
consumed += 1
obj_count += 1
yield obj
if not obj_count:
skipped += query_copy.count()
else:
skipped += obj_count
def limit(self, value):
return self[:value]
def offset(self, value):
return self[value:]
[docs] def count(self):
"""
Return the total number of rows this QueryChain's queries would return.
"""
return sum(q.count() for q in self.queries)
def __getitem__(self, key):
if isinstance(key, slice):
return self.__class__(
queries=self.queries,
limit=key.stop if key.stop is not None else self._limit,
offset=key.start if key.start is not None else self._offset
)
else:
for obj in self[key:1]:
return obj
def __repr__(self):
return '<QueryChain at 0x%x>' % id(self)