How List Attributes are Created
Attribute lists can be created directly by an FME reader, or they can
be added to a feature by a transformer.
List Attributes Created by Readers
Certain FME readers make use of
attribute lists to represent recurring values read from the original data
source, particularly when the number of such values is not known ahead
of time. In these cases, the unqualified list can be exposed in FME Workbench,
and optionally a number of elements can be explicitly exposed and then
treated as normal attributes.
For example, the DGN format returns IGDS linkages in
list form.
Open the reader Feature Type dialog and click the Format Attributes tab:
Locate the igds_linkage
attributes and enable the checkboxes:
Click OK. The attributes are added to the feature type:
List Attributes Created by Transformers
Many transformers create attribute lists as a result of their processing. For example:
- When the Joiner
is used to fetch records related to a feature from a database, and the
relation is 1 to many, each of the rows fetched are added toa structured
list on the feature. For a detailed example, see Understanding List Attributes.
- Overlay
transformations can optionally create lists of the attributes of the original
features that were involved in creating each resultant feature. For an example, see Creating Lists Using Transformers in the FME Knowledge Center.
- When the Intersector
notices overlapping line segments, it makes a list of all the attributes
of the overlapping lines and adds this to the output segment.
- Both the
Aggregator
and ListBuilder
may make lists of the attributes of features that they combine. The lists
that are created by transformers such as these can contain very important
information to be used in later processing.