Hi,
I’ve been reading the app-schema documentation end to end
and I’m left wondering about the mappingName construct… it is being cited a few times in the
docs, and I believe I have the gist of why it’s there (to handle polymorphic mappings or cases where the same nested type is used in different places), but there is no real example of it being used.
To make sure I’ve used “git grep mappingName” in the docs and the result is:
source/data/app-schema/feature-chaining.rst: * the nested element name, which is normally the targetElement or mappingName of the corresponding type.
source/data/app-schema/feature-chaining.rst:This is when the optional mappingName tag mentioned in :ref:app-schema.mapping-file comes in.
source/data/app-schema/feature-chaining.rst:Instead of passing in the nested feature type’s targetElement in the containing type’s linkElement, specify the corresponding mappingName.
source/data/app-schema/feature-chaining.rst: * The mappingName is namespace aware and case sensitive.
source/data/app-schema/feature-chaining.rst: * When the referred mappingName contains special characters such as ‘-’, it must be enclosed with single quotes in the linkElement. E.g. ‘observation-method’.
source/data/app-schema/feature-chaining.rst: * Each mappingName must be unique against other mappingName and targetElement tags across the application.
source/data/app-schema/feature-chaining.rst: * The mappingName is only to be used to identify the chained type from the nesting type. It is not a solution for multiple FeatureTypeMapping instances where > 1 of them can be queried as top level features.
source/data/app-schema/mapping-file.rst: mappedfeature1
source/data/app-schema/mapping-file.rst:* mappingName is an optional tag, to identify the mapping in :ref:app-schema.feature-chaining when there are multiple FeatureTypeMapping instances for the same type. This is solely for feature chaining purposes, and would not
source/data/app-schema/polymorphism.rst:Note: NumericType here is a mappingName, whereas gsml:CGI_TermValue is a targetElement.
source/data/app-schema/polymorphism.rst: NumericType
source/data/app-schema/polymorphism.rst:Otherwise, if NUMERIC_VALUE is less or equal than 1000, it would be encoded with attributes from FeatureTypeMapping with ‘numeric_value’ mappingName.
source/data/app-schema/polymorphism.rst:* value-n: value expression which translates to a mappingName or targetElement
source/data/app-schema/polymorphism.rst: * When specifying a mappingName or targetElement as a value in functions, make sure they’re enclosed in single quotes.
The feature chaining page is the most baffling one, it has a set of example mapping files
and I was expecting to find some usage of it in there, since the parts talking about mappingName
refer to the gsml:CGI_TermValue usage, but it’s not actually there.
The polymorphism page actually cites it in a snippet, but the context is different, it’s about avoiding extra
queries on the database. I’ve read it a few times but was not able to make head or tails of it, until I’ve found
a worked out example in the GeoServer app-schema tests:
https://github.com/geoserver/geoserver/blob/master/src/extension/app-schema/app-schema-test/src/test/resources/test-data/CGI_PlanarOrientation.xml
So, if I get the idea correctly (please correct me if I’m wrong):
- PlanarOrientation has a aziumth value that’s polymorphic, it can be either a gsml:CGI_NumericValue or a gsml:CGI_TermValue (and the same happens for the “dip” attribute)
- The property file contains all the information needed as a flat table, two columns to represent each case, one will be populated, the other null
- The linkElement uses a function to decide which mappingName to use depending on which attribute is populated
- The use of mapping name is necessary because we have literally 5 different type mappings using the same source table
- Using the mappings this way will make app schema read the table just once, without self joins, and map out the different properties as needed into the target xml document
So this example would be “using mappingName to go from a flat table to a document with nested polymorphic types”.
And then there is the case of no polymorphism, but having the same type used in several different places, it’s not polymorphism, it’s
just that the schema decided to roll out a custom type for something as common as a string for example, in order to be able to scope it with a class/namespace.
This case is instead seems to be exemplified in the geologicunit/cgi_termvalue case:
https://github.com/geoserver/geoserver/blob/master/src/extension/app-schema/app-schema-test/src/test/resources/test-data/DataReferenceData/gsml_GeologicUnit/gsml_GeologicUnit.xml#L215
In this case we have three different term value attributes in geologic unit, each one is associated to a different mapping in the second file,
each one having the same structure, but reading from a different table of the database.
Finally I beleive there would be a third case which is similar to the above one, again usage of the same type in different places, but with all the terms located in a single table instead of 3, and
having an attribute that can be used to tell what role the term plays. I haven’t found an example like it in the tests though… have I missed it?
Looking forward to your feedback, if the above is more or less correct I’d like to make a pull request adding these examples in the docs.
Cheers
Andrea
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