com.db4o.query
Interface Query

public interface Query

handle to a node in a S.O.D.A. query graph.

A node in the query graph can represent multiple classes, one class or an attribute of a class.

The graph is automatically extended with attributes of added constraints (see constrain(java.lang.Object)) and upon calls to descend(java.lang.String) that request nodes that do not yet exist.

References to joined nodes in the query graph can be obtained by "walking" along the nodes of the graph with the method descend(java.lang.String).

execute() evaluates the entire graph against all persistent objects.

execute() can be called from any Query node of the graph. It will return an ObjectSet filled with objects of the class/classes that the node, it was called from, represents.

Note:
Native queries are the recommended main query interface of db4o.

Method Summary

Constraint	constrain(java.lang.Object constraint) adds a constraint to this node.
Constraints	constraints() returns a Constraints object that holds an array of all constraints on this node.
Query	descend(java.lang.String fieldName) returns a reference to a descendant node in the query graph.
<T> ObjectSet<T>	execute() executes the Query.
Query	orderAscending() adds an ascending ordering criteria to this node of the query graph.
Query	orderDescending() adds a descending order criteria to this node of the query graph.
Query	sortBy(java.util.Comparator comparator) Sort the resulting ObjectSet by the given comparator.
Query	sortBy(QueryComparator<?> comparator) Sort the resulting ObjectSet by the given comparator.

Method Detail

constrain

Constraint constrain(java.lang.Object constraint)

adds a constraint to this node.

If the constraint contains attributes that are not yet present in the query graph, the query graph is extended accordingly.

Special behaviour for:

• class Class: confine the result to objects of one class or to objects implementing an interface.
• interface Evaluation: run evaluation callbacks against all candidates.

Parameters:

constraint - the constraint to be added to this Query.

Returns:

Constraint a new Constraint for this query node or null for objects implementing the Evaluation interface.

constraints

Constraints constraints()

returns a Constraints object that holds an array of all constraints on this node.

Returns:

Constraints on this query node.

descend

Query descend(java.lang.String fieldName)

returns a reference to a descendant node in the query graph.

If the node does not exist, it will be created.

All classes represented in the query node are tested, whether they contain a field with the specified field name. The descendant Query node will be created from all possible candidate classes.

Parameters:

fieldName - path to the descendant.

Returns:

descendant Query node

execute

<T> ObjectSet<T> execute()

executes the Query.

Returns:

ObjectSet - the result of the Query.

orderAscending

Query orderAscending()

adds an ascending ordering criteria to this node of the query graph.

If multiple ordering criteria are applied, the chronological order of method calls is relevant: criteria created by 'earlier' calls are considered more significant, i.e. 'later' criteria only have an effect for elements that are considered equal by all 'earlier' criteria.

As an example, consider a type with two int fields, and an instance set {(a:1,b:3),(a:2,b:2),(a:1,b:2),(a:2,b:3)}. The call sequence [orderAscending(a), orderDescending(b)] will result in [(a:1,b:3),(a:1,b:2),(a:2,b:3),(a:2,b:2)].

Returns:

this Query object to allow the chaining of method calls.

orderDescending

Query orderDescending()

adds a descending order criteria to this node of the query graph.

For semantics of multiple calls setting ordering criteria, see orderAscending().

Returns:

this Query object to allow the chaining of method calls.

sortBy

Query sortBy(QueryComparator<?> comparator)

Sort the resulting ObjectSet by the given comparator.

Parameters:

comparator - The comparator to apply.

Returns:

this Query object to allow the chaining of method calls.

sortBy

Query sortBy(java.util.Comparator comparator)

Sort the resulting ObjectSet by the given comparator.

Parameters:

comparator - The comparator to apply.

Returns:

this Query object to allow the chaining of method calls.