solutions
:: Tuple queries
Tuple queries
Consider a scenario in which you just want to have a list of all first names of
To solve this problem ABEL allows queries which return data in
Tuple queries and projections
The
By default, a
Tuple queries and criteria
You are restricted to use predefined criteria in tuple queries, because agent criteria expect an object and not a tuple. You can still combine them with logical operators, and even include a predefined criterion on an attribute that is not present in the projection list. These attributes will be loaded internally to check if the object satisfies the criterion, and then they are discarded for the actual result.
Example
explore_tuple_queries
-- See what can be done with tuple queries.
local
query: PS_TUPLE_QUERY [CHILD]
transaction: PS_TRANSACTION
projection: ARRAYED_LIST [STRING]
do
-- Tuple queries are very similar to normal queries.
-- I.e. you can query for CHILD objects by creating
-- a PS_TUPLE_QUERY [CHILD]
create query.make
-- It is also possible to add criteria. Agent criteria
-- are not supportedfor tuple queries however.
-- Lets search for CHILD objects with last name Doe.
query.set_criterion (criterion_factory ("last_name", criterion_factory.equals, "Doe"))
-- The big advantage of tuple queries is that you can
-- define which attributes should be loaded.
-- Thus you can avoid loading a whole object graph
-- if you're just interested in e.g. the first name.
create projection.make_from_array (<<"first_name">>)
query.set_projection (projection)
-- Execute the tuple query.
repository.execute_tuple_query (query)
-- The result of the query is a TUPLE containing the
-- requested attribute.
print ("Print all first names using a tuple query:%N")
across
query as cursor
loop
-- It is possible to downcast the TUPLE
-- to a tagged tuple with correct type.
check attached {TUPLE [first_name: STRING]} cursor.item as tuple then
print (tuple.first_name + "%N")
end
end
-- Cleanup and error handling.
query.close
if query.has_error then
print ("An error occurred!%N")
end
end