An app-schema feature type is configured using a mapping file that defines the data source for the feature and the mappings from the source data to XPaths in the output XML.
Here is an outline of a mapping file:
<?xml version="1.0" encoding="UTF-8"?> <as:AppSchemaDataAccess xmlns:as="http://www.geotools.org/app-schema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.geotools.org/app-schema AppSchemaDataAccess.xsd"> <namespaces>...</namespaces> <includedTypes>...</includedTypes> <sourceDataStores>...</sourceDataStores> <catalog>...</catalog> <targetTypes...</targetTypes> <typeMappings>...</typeMappings> </as:AppSchemaDataAccess>
- namespaces defines all the namespace prefixes used in the mapping file.
- includedTypes (optional) defines all the included non-feature type mapping file locations that are referred in the mapping file.
- sourceDataStores provides the configuration information for the source data stores.
- catalog is the location of the OASIS Catalog used to resolve XML Schema locations.
- targetTypes is the location of the XML Schema that defines the feature type.
- typeMappings give the relationships between the fields of the source data store and the elements of the output complex feature.
Mapping file schema¶
- AppSchemaDataAccess.xsd is optional because it is not used by GeoServer. The presence of AppSchemaDataAccess.xsd in the same folder as the mapping file enables XML editors to observe its grammar and provide contextual help.
The namespaces section defines all the XML namespaces used in the mapping file:
<Namespace> <prefix>gsml</prefix> <uri>urn:cgi:xmlns:CGI:GeoSciML:2.0</uri> </Namespace> <Namespace> <prefix>gml</prefix> <uri>http://www.opengis.net/gml</uri> </Namespace> <Namespace> <prefix>xlink</prefix> <uri>http://www.w3.org/1999/xlink</uri> </Namespace>
Non-feature types (eg. gsml:CompositionPart is a data type that is nested in gsml:GeologicUnit) may be mapped separately for its reusability, but we don’t want to configure it as a feature type as we don’t want to individually access it. Related feature types don’t need to be explicitly included here as it would have its own workspace configuration for GeoServer to find it. The location path in Include tag is relative to the mapping file. For an example, if gsml:CompositionPart configuration file is located in the same directory as the gsml:GeologicUnit configuration:
<includedTypes> <Include>gsml_CompositionPart.xml</Include> </includedTypes>
Every mapping file requires at least one data store to provide data for features. app-schema reuses GeoServer data stores, so there are many available types. See Data Stores for details of data store configuration. For example:
<sourceDataStores> <DataStore> <id>datastore</id> <parameters> ... </parameters> </DataStore> ... </sourceDataStores>
If you have more than one DataStore in a mapping file, be sure to give them each a distinct id.
The location of an OASIS XML Catalog configuration file, given as a path relative to the mapping file. See Application Schema Resolution for more information. For example:
The targetTypes section lists all the application schemas required to define the mapping. Typically only one is required. For example:
<targetTypes> <FeatureType> <schemaUri>http://www.geosciml.org/geosciml/2.0/xsd/geosciml.xsd</schemaUri> </FeatureType> </targetTypes>
typeMappings and FeatureTypeMapping¶
The typeMappings section is the heart of the app-schema module. It defines the mapping from simple features to the the nested structure of one or more simple features. It consists of a list of FeatureTypeMapping elements, which each define one output feature type. For example:
<typeMappings> <FeatureTypeMapping> <mappingName>mappedfeature1</mappingName> <sourceDataStore>datastore</sourceDataStore> <sourceType>mappedfeature</sourceType> <targetElement>gsml:MappedFeature</targetElement> <attributeMappings> <AttributeMapping> ...
mappingName is an optional tag, to identify the mapping in Feature Chaining when there are multiple FeatureTypeMapping instances for the same type. This is solely for feature chaining purposes, and would not work for identifying top level features.
sourceDataStore must be an identifier you provided when you defined a source data store the sourceDataStores section.
sourceType is the simple feature type name. For example:
- a table or view name, lowercase for PostGIS, uppercase for Oracle.
- a property file name (without the .properties suffix)
targetElement is the the element name in the target application schema. This is the same as the WFS feature type name.
attributeMappings and AttributeMapping¶
attributeMappings comprises a list of AttributeMapping elements:
<AttributeMapping> <targetAttribute>...</targetAttribute> <idExpression>...</idExpression> <sourceExpression>...</sourceExpression> <targetAttributeNode>...</targetAttributeNode> <isMultiple>...</isMultiple> <ClientProperty>...</ClientProperty> </AttributeMapping>
targetAttribute is the XPath to the output element, in the context of the target element. For example, if the containing mapping is for a feature, you should be able to map a gml:name property by setting the target attribute:
Multivalued attributes resulting from Denormalised sources are automatically encoded. If you wish to encode multivalued attributes from different input columns as a specific instance of an attribute, you can use a (one-based) index. For example, you can set the third gml:name with:
The reserved name FEATURE_LINK is used to map data that is not encoded in XML but is required for use in Feature Chaining.
A CQL expression that is used to set the gml:id of the output feature type. This could be a column in a database, the automatically generated simple feature ID obtained with getId(), or some other expression.
Every feature type must have one idExpression mapping to set its gml:id. This requirement is an implementation limitation (strictly, gml:id is optional in GML).
gml:id must be an NCName.
Use a sourceExpression tag to set the element content from source data. For example, to set the element content from a column called DESCRIPTION:
If sourceExpression is not present, the generated element is empty (unless set by another mapping).
You can use CQL expressions to calculate the content of the element. This example concatenated strings from two columns and a literal:
<sourceExpression> <OCQL>strConCat(FIRST , strConCat(' followed by ', SECOND))</OCQL> </sourceExpression>
You can also use CQL expressions for vocabulary translations. Read more about it in Vocabulary functions.
Avoid use of CQL expressions for properties that users will want to query, because the current implementation cannot reverse these expressions to generate efficient SQL, and will instead read all features to calculate the property to find the features that match the filter query. Falling back to brute force search makes queries on CQL-calculated expressions very slow. If you must concatenate strings to generate content, you may find that doing this in your database is much faster.
targetAttributeNode is required wherever a property type contains an abstract element and app-schema cannot determine the type of the enclosed attribute.
In this example, om:result is of xs:anyType, which is abstract. We can use targetAttributeNode to set the type of the property type to a type that encloses a non-abstract element:
<AttributeMapping> <targetAttribute>om:result</targetAttribute> <targetAttributeNode>gml:MeasureType<targetAttributeNode> <sourceExpression> <OCQL>TOPAGE</OCQL> </sourceExpression> <ClientProperty> <name>xsi:type</name> <value>'gml:MeasureType'</value> </ClientProperty> <ClientProperty> <name>uom</name> <value>'http://www.opengis.net/def/uom/UCUM/0/Ma'</value> </ClientProperty> </AttributeMapping>
If the casting type is complex, the specific type is implicitly determined by the XPath in targetAttribute and targetAttributeNode is not required. E.g., in this example om:result is automatically specialised as a MappedFeatureType:
<AttributeMapping> <targetAttribute>om:result/gsml:MappedFeature/gml:name</targetAttribute> <sourceExpression> <OCQL>NAME</OCQL> </sourceExpression> </AttributeMapping>
Although it is not required, we may still specify targetAttributeNode for the root node, and map the children attributes as per normal. This mapping must come before the mapping for the enclosed elements. By doing this, app-schema will report an exception if a mapping is specified for any of the children attributes that violates the type in targetAttributeNode. E.g.:
<AttributeMapping> <targetAttribute>om:result</targetAttribute> <targetAttributeNode>gsml:MappedFeatureType<targetAttributeNode> </AttributeMapping> <AttributeMapping> <targetAttribute>om:result/gsml:MappedFeature/gml:name</targetAttribute> <sourceExpression> <OCQL>NAME</OCQL> </sourceExpression> </AttributeMapping>
Note that the GML encoding rules require that complex types are never the direct property of another complex type; they are always contained in a property type to ensure that their type is encoded in a surrounding element. Encoded GML is always type/property/type/property. This is also known as the GML “striping” rule. The consequence of this for app-schema mapping files is that targetAttributeNode must be applied to the property and the type must be set to the XSD property type, not to the type of the contained attribute (gsml:CGI_TermValuePropertyType not gsml:CGI_TermValueType). Because the XPath refers to a property type not the encoded content, targetAttributeNode appears in a mapping with targetAttribute and no other elements when using with complex types.
The encodeIfEmpty element will determine if an attribute will be encoded if it contains a null or empty value. By default encodeIfEmpty is set to false therefore any attribute that does not contain a value will be skipped:
encodeIfEmpty can be used to bring up attributes that only contain client properties such as xlink:title.
The isMultiple element states whether there might be multiple values for this attribute, coming from denormalised rows. Because the default value is false and it is omitted in this case, it is most usually seen as:
For example, the table below is denormalised with NAME column having multiple values:
|gu.25678||Yaugher Volcanic Group 1||Olivine basalt, tuff, microgabbro|
|gu.25678||Yaugher Volcanic Group 2||Olivine basalt, tuff, microgabbro|
The configuration file specifies isMultiple for gml:name attribute that is mapped to the NAME column:
<AttributeMapping> <targetAttribute>gml:name</targetAttribute> <sourceExpression> <OCQL>NAME</OCQL> </sourceExpression> <isMultiple>true</isMultiple> <ClientProperty> <name>codeSpace</name> <value>'urn:ietf:rfc:2141'</value> </ClientProperty> </AttributeMapping>
The output produces multiple gml:name attributes for each feature grouped by the id:
<gsml:GeologicUnit gml:id="gu.25678"> <gml:description>Olivine basalt, tuff, microgabbro</gml:description> <gml:name codeSpace="urn:ietf:rfc:2141">Yaugher Volcanic Group 1</gml:name> <gml:name codeSpace="urn:ietf:rfc:2141">Yaugher Volcanic Group 2</gml:name> ...
The isList element states whether there might be multiple values for this attribute, concatenated as a list. The usage is similar with isMultiple, except the values appear concatenated inside a single node instead of each value encoded in a separate node. Because the default value is false and it is omitted in this case, it is most usually seen as:
For example, the table below has multiple POSITION for each feature:
The configuration file uses isList on timePositionList attribute mapped to POSITION column:
<AttributeMapping> <targetAttribute>csml:timePositionList</targetAttribute> <sourceExpression> <OCQL>POSITION</OCQL> </sourceExpression> <isList>true</isList> </AttributeMapping>
The output produced:
<csml:pointSeriesDomain> <csml:TimeSeries gml:id="ID1.2"> <csml:timePositionList>1949-05 1949-06 1949-07 1949-08 1949-09</csml:timePositionList> </csml:TimeSeries> </csml:pointSeriesDomain>
ClientProperty (optional, multivalued)¶
A mapping can have one or more ClientProperty elements which set XML attributes on the mapping target. Each ClientProperty has a name and a value that is an arbitrary CQL expression. No OCQL element is used inside value.
This example of a ClientProperty element sets the codeSpace XML attribute to the literal string urn:ietf:rfc:2141. Note the use of single quotes around the literal string. This could be applied to any target attribute of GML CodeType:
<ClientProperty> <name>codeSpace</name> <value>'urn:ietf:rfc:2141'</value> </ClientProperty>
When the GML association pattern is used to encode a property by reference, the xlink:href attribute is set and the element is empty. This ClientProperty element sets the xlink:href XML attribute to to the value of the RELATED_FEATURE_URN field in the data source (for example, a column in an Oracle database table). This mapping could be applied to any property type, such a gml:FeaturePropertyType, or other type modelled on the GML association pattern:
<ClientProperty> <name>xlink:href</name> <value>RELATED_FEATURE_URN</value> </ClientProperty>
See the discussion in Feature Chaining for the special case in which xlink:href is created for multivalued properties by reference.
String literals are enclosed in single quotes, for example 'urn:ogc:def:nil:OGC:missing'.
The uDig manual contains information on CQL:
When referring to database table/view names or column names, use:
- lowercase for PostGIS
- UPPERCASE for Oracle Spatial and ArcSDE
Multivalued properties from denormalised sources (the same source feature ID appears more than once) are automatically encoded. For example, a view might have a repeated id column with varying name so that an arbitrarily large number of gml:name properties can be encoded for the output feature.
Denormalised sources must grouped so that features with duplicate IDs are provided without any intervening features. This can be achieved by ensuring that denormalised source features are sorted by ID. Failure to observe this restriction will result in data corruption. This restriction is however not necessary when using Joining Support For Performance because then ordering will happen automatically.