Everyday Gaelyk: Common Query DSL pitfalls

03 duben 2013

Gaelyk Query DSL makes quering
Google App Engine Datastore a lot simpler but sometimes
it isn't working as expected.

Variable naming conflict

Some of problems origins from using the variable or property names which are already taken.
Names of the properties in where clause are converted to String automatically. What property we will be querying?

Variable name conflict

def count = 123
// a lot of code here so you've already forgotten you have such a variable

def maxCount = 100

datastore.execute {
    from Item
    where count <= maxCount
}

This query will be translated as 123 <= 100 which probably isn't what you wanted.

Binding variable name conflict

Previous example was quite obvious but what if we have following query:

Binding variable name conflict

datastore.execute {
    from Item
    where users > 10
}

Instead of getting result of items having more than ten users we get empty result set. It's
because users is Gaelyk's shortcut to UserService
so the where clause is translated to something like UserService@xyz123 > 10 which obviously returns no results.

If you run into name conflict just use good old String as property name in the where clause such as where 'users' > 10.

Entity pitfalls

@Entity annotation adds sevral useful methods to the POGO class such as findAll which resemble their
Grails counterparts. But don't get confused. The syntax of
using such methods differs slightly. find, findAll or count method are just shortcuts to Query DSL!

Using findAll method

@Entity class Item {
    int count
}

Item.findAll { count == 10 }

The query listed above will return all the items since the condition is ignored
because the where keyword is missing.

Using findAll method with where

@Entity class Item {
    int count
}

Item.findAll { where count == 10 }

We have added the where keyword to the findAll method but now we'll always get empty result because all field of @Entity class
are unindexed by default. You need to mark the field @Indexed to use it in queries.

Using findAll method with where on indexed field

@Entity class Item {
    @Indexed int count
}

Item.findAll { where count == 10 }

Summary

Using Query DSL you should basically always take care about two important things:

  1. Property names are always converted to String, this is may make mess if the variable with the same name is already defined (even in Gaelyk shortcut bindings). Use String constants instead of property names in case of any possible name conflict.
  2. All properties you're querying must be indexed. When using @Entity annotations all the properties are unindexed by default.