Query (Google App Engine API for Java)

com.google.appengine.api.datastore

Class Query

  • java.lang.Object
    • com.google.appengine.api.datastore.Query
  • All Implemented Interfaces:
    java.io.Serializable


    public final class Query
    extends java.lang.Object
    implements java.io.Serializable
    Query encapsulates a request for zero or more Entity objects out of the datastore. It supports querying on zero or more properties, querying by ancestor, and sorting. Entity objects which match the query can be retrieved in a single list, or with an unbounded iterator.
    See Also:
    Serialized Form
    • Field Detail

      • KIND_METADATA_KIND

        @Deprecated
        public static final java.lang.String KIND_METADATA_KIND
        Deprecated. 
        See Also:
        Constant Field Values
      • PROPERTY_METADATA_KIND

        @Deprecated
        public static final java.lang.String PROPERTY_METADATA_KIND
        Deprecated. 
        See Also:
        Constant Field Values
      • NAMESPACE_METADATA_KIND

        @Deprecated
        public static final java.lang.String NAMESPACE_METADATA_KIND
        Deprecated. 
        See Also:
        Constant Field Values
    • Constructor Detail

      • Query

        public Query()
        Create a new kindless Query that finds Entity objects. Note that kindless queries are not yet supported in the Java dev appserver. Currently the only operations supported on a kindless query are filter by __key__, ancestor, and order by __key__ ascending.
      • Query

        public Query(java.lang.String kind)
        Create a new Query that finds Entity objects with the specified kind. Note that kindless queries are not yet supported in the Java dev appserver.
        Parameters:
        kind - the kind or null to create a kindless query
      • Query

        public Query(Key ancestor)
        Create a new Query that finds Entity objects with the specified Key as an ancestor.
        Parameters:
        ancestor - the ancestor key or null
        Throws:
        java.lang.IllegalArgumentException - If ancestor is not complete.
      • Query

        public Query(java.lang.String kind,
                     Key ancestor)
        Create a new Query that finds Entity objects with the specified kind and the specified ancestor. Note that kindless queries are not yet supported in the Java dev appserver.
        Parameters:
        kind - the kind or null to create a kindless query
        ancestor - the ancestor key or null
        Throws:
        java.lang.IllegalArgumentException - If the ancestor is not complete.
    • Method Detail

      • getKind

        public java.lang.String getKind()
        Only Entity objects whose kind matches this value will be returned.
      • getAppId

        public java.lang.String getAppId()
        Returns the appId for this Query.
      • getNamespace

        public java.lang.String getNamespace()
        Returns the namespace for this Query.
      • getAncestor

        public Key getAncestor()
        Gets the current ancestor for this query, or null if there is no ancestor specified.
      • setAncestor

        public Query setAncestor(Key ancestor)
        Sets an ancestor for this query. This restricts the query to only return result entities that are descended from a given entity. In other words, all of the results will have the ancestor as their parent, or parent's parent, or etc. If null is specified, unsets any previously-set ancestor. Passing null as a parameter does not query for entities without ancestors (this type of query is not currently supported).
        Returns:
        this (for chaining)
        Throws:
        java.lang.IllegalArgumentException - If the ancestor key is incomplete, or if you try to unset an ancestor and have not set a kind, or if you try to unset an ancestor and have not previously set an ancestor.
      • setDistinct

        public Query setDistinct(boolean distinct)
        Parameters:
        distinct - if this query should be distinct. This may only be used when the query has a projection.
        Returns:
        this (for chaining)
      • getDistinct

        public boolean getDistinct()
        Returns:
        if this query is distinct
        See Also:
        setDistinct(boolean)
      • addSort

        public Query addSort(java.lang.String propertyName)
        Specify how the query results should be sorted. The first call to addSort will register the property that will serve as the primary sort key. A second call to addSort will set a secondary sort key, etc. This method will always sort in ascending order. To control the order of the sort, use addSort(String,SortDirection). Note that entities with multi-value properties identified by propertyName will be sorted by the smallest value in the list. For more information on sorting properties with multiple values please see the datastore documentation.
        Returns:
        this (for chaining)
        Throws:
        java.lang.NullPointerException - If any argument is null.
      • addSort

        public Query addSort(java.lang.String propertyName,
                             Query.SortDirection direction)
        Specify how the query results should be sorted. The first call to addSort will register the property that will serve as the primary sort key. A second call to addSort will set a secondary sort key, etc. Note that if direction is Query.SortDirection.ASCENDING, entities with multi-value properties identified by propertyName will be sorted by the smallest value in the list. If direction is Query.SortDirection.DESCENDING, entities with multi-value properties identified by propertyName will be sorted by the largest value in the list. For more information on sorting properties with multiple values please see the datastore documentation.
        Returns:
        this (for chaining)
        Throws:
        java.lang.NullPointerException - If any argument is null.
      • getSortPredicates

        public java.util.List<Query.SortPredicate> getSortPredicates()
        Returns a mutable list of the current sort predicates.
      • setKeysOnly

        public Query setKeysOnly()
        Makes this query fetch and return only keys, not full entities.
        Returns:
        this (for chaining)
      • clearKeysOnly

        public Query clearKeysOnly()
        Clears the keys only flag.
        Returns:
        this (for chaining)
        See Also:
        setKeysOnly()
      • addProjection

        public Query addProjection(Projection projection)
        Adds a projection for this query.

        Projections are limited in the following ways:

        • Un-indexed properties cannot be projected and attempting to do so will result in no entities being returned.
        • Projection names must be unique.
        • Properties that have an equality filter on them cannot be projected. This includes the operators Query.FilterOperator.EQUAL and Query.FilterOperator.IN.
        Parameters:
        projection - the projection to add
        Returns:
        this (for chaining)
        Throws:
        java.lang.IllegalArgumentException - if the query already contains a projection with the same name
        See Also:
        getProjections()
      • getProjections

        public java.util.Collection<Projection> getProjections()
        Returns a mutable collection properties included in the projection for this query.

        If empty, the full or keys only entities are returned. Otherwise partial entities are returned. A non-empty projection is not compatible with setting keys-only. In this case a IllegalArgumentException will be thrown when the query is prepared.

        Projection queries are similar to SQL statements of the form:

        SELECT prop1, prop2, ...
        As they return partial entities, which only contain the properties specified in the projection. However, these entities will only contain a single value for any multi-valued property and, if a multi-valued property is specified in the order, an inequality property, or the projected properties, the entity will be returned multiple times. Once for each unique combination of values.

        Specifying a projection:

        • May change the type of any property returned in a projection.
        • May change the index requirements for the given query.
        • Will cause a partial entity to be returned.
        • Will cause only entities that contain those properties to be returned.
        However, projection queries are significantly faster than normal queries.
        Returns:
        a mutable collection properties included in the projection for this query
        See Also:
        addProjection(Projection)
      • isKeysOnly

        public boolean isKeysOnly()
        Returns true if this query will fetch and return keys only, false if it will fetch and return full entities.
      • reverse

        public Query reverse()
        Creates a query sorted in the exact opposite direction as the current one. This function requires a sort order on Entity.KEY_RESERVED_PROPERTY to guarantee that each entity is uniquely identified by the set of properties used in the sort (which is required to exactly reverse the order of a query). Advanced users can reverse the sort orders manually if they know the set of sorted properties meets this requirement without a order on Entity.KEY_RESERVED_PROPERTY. The results of the reverse query will be the same as the results of the forward query but in reverse direction. Cursors from the original query may also be used in the reverse query.
        Returns:
        A new query with the sort order reversed.
        Throws:
        java.lang.IllegalStateException - if the current query is not sorted by Entity.KEY_RESERVED_PROPERTY.
      • equals

        public boolean equals(java.lang.Object o)
        Overrides:
        equals in class java.lang.Object
      • hashCode

        public int hashCode()
        Overrides:
        hashCode in class java.lang.Object
      • toString

        public java.lang.String toString()
        Outputs a SQL like string representing the query.
        Overrides:
        toString in class java.lang.Object

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