The Extended by relation declares a file to be an extension of another file. The relation records an association that is not expressed by any other relation, and is, in particular, a one-to-one or one-to-none association between the identifiers of two files.
When it is used with an existing Owned by relation, the Extended by relation has no effect on the entries of the file being extended; it merely makes the fields from the extended file available for selection as virtual fields on the file being extended.
A virtual field is logically present in a view of a file, though it physically resides in another file.
See the section Adding Virtual Fields to File to File Relations for more information.
A one-to-none relation denotes a relation where an instance exists in one file and the corresponding instance does not exist in another file. For example, the Product file could be extended by the export details file. Some products may be exported and some may not. This would mean that a product record may or may not have an associated record in the export detail file.
Note: It is not recommended that you use the Extended by relation unless the extending file is Owned by the extended file, as unpredictable results may occur during source code generation.
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