Package com.db4o.query

Interface Query

  • All Known Implementing Classes:
    QQuery
    public interface Query
    Handle to a node in a S.O.D.A. query graph. NOTE: Soda queries silently ignore invalid specified fields, constraints etc.

    A node in the query graph can represent a class or an attribute of a class.

    The graph is automatically extended with attributes of added constraints (see constrain(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(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:
    ObjectContainer.query(Predicate) Native queries} are the recommended main query interface of db4o.

    • 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.
        • Evaluation-instance: run evaluation callbacks against all candidates.
        Parameters:
        constraint - the constraint to be added to this Query.
        Returns:
        a new Constraint for this query node or null for objects implementing the Evaluation interface.

      • 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

      • 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.

        Ordering by non primitive fields works only for classes that implement the Activatable interface and TransparentActivationSupport is enabled.

        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.