QUERY

In UBIK, queries are used for fetching a list of [[Contentclass|ContentClasses] from the database. Queries use data base mechanism for perfomant data access and will deliver a list of objects of a certain Metaclass. Queries can be used in different scenarios:

• data fetching (n records)
• finding a single or a set of recordset(s)
• used as definition of root nodes in a View

A Query is defined for a single MetaClass. The MetaClass is defined by a Referenceproperty (“FILTEROBJECT”). A Query will deliver ContentClasses(instances) of the defined MetaClass. A Query holds a list of QueryItems. A single QueryItem defines a single filter rule for the evaluation of the result collection:

• OPERATOR

e.g. “=”

• LIKECOMPARISON

is the given item a “LIKE” filter criteria (wildcard is added to the filtervalue)

• FILTERVALUE

the value the objects are compared to

• ISNULL

searched for NULL values

• LOGICALAND

is there a logical AND relation between the given QueryItem and its predecessor (TRUE) or an OR relation (false)

The correspondence between Queries and its items is created and stored by a relationship (“QUERYRELATION”). Therefore a single QueryItem can be used several times for different Queries.

We often face the need to fetch a list of objects meeting arbitary criteria, for example for

• scanning of ID markers (QR-Code, Barcode, …)
• show objects from deeper hierarchy levels

Storing of data permanently on the device would need too much resources, hence we load the objects from server on demand. Characteristics of a content query are

• derivative from QUERY classified as SYSCLS_QUERY marks instances as content queries
• Related to instances via relations
• Placed in view hierarchy defined via view items
• Require context scopes on client side

Configuration of a content query requies

• Set filter object (FILTEROBJECT) similar as for other query
• Create a proper relation
• Create a query view item and set the instance as QUERY
• Create a relation view item and set the according default MetaClass and relation
• Create and relate a query context scope and set the instance as QUERY
• For the scope define the MetaProperties, which should be available as filter criteria on the client. Without appropriate query item, equality is used as filter operator (=)

It is also possible to define a Database View and use this View for the evaluation of Queries (see VirtualContentClass).