datastore query 问题

[timshi]

假设我有这样一个 model

class Person{
@Persistent
private List<Category> tags = ArrayList<Category>()
}

I want to let the user query a person based on his/her tag, so I had my query filter like this:

tags.contains(tagValue1)

and if the user want to search for multiple tags, I would just add to the filter so if the user is searching for 3 tags, then the query would be

tags.contains(tagValue1) && tags.contains(tagValue2) && tags.contains(tagValue3)

I think this approach is wrong, because the datastore then needs to have an index that have the tags property three times... and if the user search for more than 3 tags at a time then it will be broken.

What's the proper way to do this? Do you guys have any suggestions?

[MarkFull]

这个是3个queries吧

[timshi]

你是说我必须做3个query然后再intersect the result myself?

[MarkFull]

intersect是下策，built-in的GQL我不是很熟悉

[timshi]

那你会怎么做？ 我本来以为这个个index应该是这样的

person1, tagValue1
person1, tagValue2
person1, tagValue3

i.e an additional for each value in the multi-value property, and GAE should do a merge join to get me the result.

But my index file looks like this

<datastore-index kind="Person" ancestor="false" source="auto">
<property name="tags" direction="asc"/>
<property name="tags" direction="asc"/>
<property name="tags" direction="asc"/>
<property name="tags" direction="asc"/>
.....

[MarkFull]

no, each property should specify once for the same kind (Person).
There should be only one line of <property name="tags" direction="asc"/>

This is the GQL format you can reference:
SELECT [* | __key__] FROM <kind>
[WHERE <condition> [AND <condition> ...]]
[ORDER BY <property> [ASC | DESC] [, <property> [ASC | DESC] ...]]
[LIMIT [<offset>,]<count>]
[OFFSET <offset>]

<condition> := <property> {< | <= | > | >= | = | != } <value>
<condition> := <property> IN <list>
<condition> := ANCESTOR IS <entity or key>

[MarkFull]

Instead of using GQL, maybe you can use this:

filter(property_operator, value)
Adds a property condition filter to the query. Only entities with properties that meet all of the conditions will be returned by the query.
Arguments:
property_operator
A string containing the property name, and an optional comparison operator. The name and the operator must be separated by a space, as in: age > The following comparison operators are supported: < <= = >= > != IN If the operator is omitted from the string (the argument is just the property name), the filter uses the = operator.
value
The value to use in the comparison on the right-hand side of the expression. Its type should be the value data type for the property being compared. See Types and Property Classes.
query.filter('height >', 42).filter('city = ', 'Seattle')

query.filter('user = ', users.get_current_user())

from http://code.google.com/appengine/docs/python/datastore/queryclass.html#Query_filter