Feature Chaining

Feature Chaining is one of the more complex aspects of GeoServer App Schema configuration. It is utilized in those cases where a nested featureType or dataType can have a cardinality of more than one. In such cases, an additional database table will be required for the provision of the data pertaining to the nested featureType or dataType. Feature Chaining allows one to configure the mapping for each individual featureType or dataType separately within a FeatureTypeMapping section of the configuration file, then reference this complete feature mapping for the content of the attribute providing the nested type.

Feature Chaining of dataTypes

Provision of a chained dataType

In our example, the dataType NestedDT is provided by the nestedDtMultiple attribute of MainFT with a cardinality of 0..* . The data for the dataType NestedDT is stored in the database table ex_nesteddt. The FeatureTypeMapping for this nested dataType has the same structure as that done for the MainFT, with a few additions. In the header a mappingName is provided that allows us to reference this feature mapping.

In the example below, the mappingName is defined as '_Nested_DT'

<FeatureTypeMapping>
  <mappingName>_Nested_DT</mappingName>
  <sourceDataStore>idDataStoreInsp</sourceDataStore>
  <sourceType>ex_nesteddt</sourceType>
  <targetElement>ex:NestedDT</targetElement>

In addition, we must provide an AttributeMapping section that specifies the database column to be utilized as a foreign key to the main table; the name provided under targetAttribute for this section must be the same as used when referencing this dataType. In the example below the database column main_ft_id references the id column of the main table as specified under sourceExpression, while the name for the targetAttribute is provided as 'FEATURE_LINK'

<AttributeMapping>
  <targetAttribute>FEATURE_LINK</targetAttribute>
  <sourceExpression>
    <OCQL>main_ft_id</OCQL>
  </sourceExpression>
</AttributeMapping>

*Note: in cases where the same dataType is referenced from multiple locations, whereby different foreign keys should be utilized in associating the correct data, multiple such AttributeMapping sections can be provided, each with a different name for targetAttribute, e.g. 'FEATURE_LINK[1]' and 'FEATURE_LINK[2]'

Referencing of a chained dataType

Once the dataType to be nested has been configured as described above, one must configure the reference to it within the MainFT as shown below. The targetAttribute is the name of the XML element as in basic feature mapping. In the sourceExpression the linkage to the chained dataType is provided via the following elements:

• OCQL: the column of the main database table referenced by the foreign key of the linked feature, in our example 'id'
• linkElement: the mappingName of the FeatureTypeMapping to be linked, in our example '_Nested_DT'
• linkField: the name provided as targetAttribute of the AttributeMapping providing the correct foreign key association, in our example 'FEATURE_LINK'

<AttributeMapping>
  <targetAttribute>ex:nestedDTMultiple</targetAttribute>
  <sourceExpression>
    <OCQL>id</OCQL>
    <linkElement>_Nested_DT</linkElement>
    <linkField>FEATURE_LINK</linkField>
  </sourceExpression>
</AttributeMapping>	

Feature Chaining of featureTypes

The same principle as described above can also be applied to featureTypes. The only addition is the provision of an idExpression within the FeatureTypeMapping of the linked featureType.

<AttributeMapping>
  <targetAttribute>ex:NestedFT</targetAttribute>
  <idExpression>
    <OCQL>gmlid</OCQL>
  </idExpression>
</AttributeMapping>