I have a query and I can apply filters on them without any problem. This works fine:
query.filter('foo =', 'bar')
But what if I want to filter my query by key or a list of keys?
I have them as Key() property or as a string and by trying something like this, it didn't work:
query.filter('key =', 'some_key') #no success
query.filter('key IN', ['key1', 'key2']) #no success
Whilst it's possible to filter on key - see @dplouffe's answer - it's not a good idea. 'IN' clauses execute one query for each item in the clause, so you end up doing as many queries as there are keys, which is a particularly inefficient way to achieve your goal.
Instead, use a batch fetch operation, as @Luke documents, then filter any elements you don't want out of the list in your code.