如何编写对字段长度进行过滤的gql查询？

Question

I want to filter a query by the length of one of the fields in a datastore but it appears I cannot include a len function in the WHERE clause.

q = db.GqlQuery("SELECT CommentCode FROM Comments " + 
                "WHERE RefObjType = 'paper' AND RefObjID = :1 AND len(CommentCode) = :2" +
                "ORDER BY CommentCode DESC ", RefObjID, 1)

This results in: BadQueryError: Parse Error: Invalid WHERE Condition at symbol (

Can anyone give me an example of how I might get this to work?

The RefObjID part works because the entire query works if I just leave out the len() filter.

Thanks.

Answer 1

You can't (at least as far I understand it) because of how the DataStore works. The length of the string needs to be a property (either static or dynamic using db.Expando ) of your Model , and then you filter on that.

Answer 2

A possible solution:
Modify the query to retrieve the results in DESC order and then process the results by accessing the required result after the query retrieval.

Change query:

  q = db.GqlQuery("SELECT CommentCode FROM Comments " + 
                    "WHERE RefObjType = 'paper' AND RefObjID = :1" +
                    "ORDER BY CommentCode DESC ", RefObjID, 1)

After this select the rows that have length(CommentCode)==2 with a loop and a conditional statement.