ListBasedFeatureMerger

Merges the attributes and/or geometry of one set of features onto another set of features, based on matching list attribute values with key attribute values and expressions.

Jump to Configuration

Typical Uses

Combining attributes and/or geometry from two different streams of features, based on a common key attribute value or expression, where the key of the receiving feature (Requestor) is stored as a list attribute.
Referencing a lookup table to a list attribute

How does it work?

The ListBasedFeatureMerger joins two feature streams based on a common key attribute or expression, where one of the key values is a list attribute. Every value in the list (on every feature) is considered for matching, as the transformer tests each list element in turn for a match.

It receives two the streams of features via its input ports.

Requestor:the features that will receive new attributes and/or geometry. The Requestor must have a list attribute that will be used as a key to join the Supplier(s).

Supplier: provide attributes and/or geometry to be merged onto the Requestors.

Matches between Requestor and Supplier are identified according to the Join Attributes configuration in the parameters dialog. A Requestor feature matches a Supplier feature when a key in the specified Requestor List Attribute (for example, list{}.key) matches the specified Supplier Attribute parameter (for example, supplierKey).

When a Requestor finds a matching Supplier, the attributes and/or geometry from the Supplier are merged with the Requestor.

The Incomplete Requestors parameter specifies whether or not to merge Suppliers onto partially matching Requestors when there are keys in the Requestor List Attribute that have no matching Suppliers. A Requestor feature is considered fully matched if every key in the Requestor List Attribute matches one or more Supplier features.

If the Requestor already has an attribute that the Supplier also has, the Requestor's original value for that attribute can be preserved or overridden.
A single Supplier may be used by many Requestors.
Many Suppliers can be merged onto a single Requestor.
When attribute names conflict, you can choose whether Requestor or Supplier attribute values are maintained by using the Conflict Resolution parameter, and whether null values follow the same behavior by using the Ignore Nulls parameter.

Examples

Example: Using a list attribute to merge features

In this example, we have a set of polygons representing neighborhoods. They have a list attribute - FoodVendors{} - that contains unique IDs (KEY) for the food trucks located in the area. The task is to merge additional attributes for each vendor from a spreadsheet.

In this portion of a workspace, the area (with list) features are routed into the Requestor port of a ListBasedFeatureMerger. The spreadsheet is attached to the Supplier port.

In the parameters dialog, the Join Attributes are set to match the FoodVendors{}.KEY list attribute (Requestor) with the KEY attribute from the spreadsheet (Supplier).

In Merge Parameters, we choose to merge Attributes Only, and (importantly) to Process Duplicate Suppliers. This will ensure that all food vendors in the list are matched, not just the first one encountered.

And finally, we enable Generate List, to create new list attributes on the output features. It will contain all matching Business names and descriptions, via Selected Attributes. Note the list name entered - FoodVendors - is the same as the existing list name. This will result in the new attributes being added to (and expanding) the current list. If we entered a different name here, a new list attribute would be created.

Inspecting the features output from the Merged port, we see that BUSINESS_NAME and DESCRIPTION have been added to the FoodVendors{} list and can now be used elsewhere in the workspace.

Usage Notes

The ListBasedFeatureMerger is a very specific instance of the FeatureMerger dealing with the case where the Requestor attribute is a list. If the requestor attribute is NOT a list, use the FeatureMerger or another joining method.

Choosing a Feature Joining Method

Many transformers can perform data joining based on matching attributes, expressions and/or geometry. When choosing one for a specific joining task, considerations include the complexity of the join, data format, indexing, conflict handling, and desired results. Some transformers use SQL syntax, and some access external databases directly. They may or may not support list attribute reading and creation.

Generally, choosing the one that is most specific to the task you need to accomplish will provide the optimal performance results. If there is more than one way to do it (which is frequently the case), time spent on performance testing alternate methods may be worthwhile. Performance may vary greatly depending on the existence of key indexes when reading external tables (as opposed to features already in the workspace).

Joining Transformers Comparison Matrix

Transformer	Match By	Uses SQL Statements	Can Create List	Input Type	Notable	Description
FeatureJoiner	Attributes	No	No	Features	Best performance for simple joins on attribute values Cannot create lists - represents multiple joins as multiple output features Cannot use constructed keys or expressions	Joins features by combining the attributes and/or geometry of features based on common key attribute values. Performs the equivalent of Inner, Left, and Full SQL joins.
FeatureMerger	Attributes	No	Yes	Features	Can use expressions as keys (constructed keys) Can create lists – represents multiple joins as a list on a single feature Cannot match on geometry	Merges the attributes and/or geometry of one set of features onto another set of features, based on matching key attribute values and expressions.
ListBasedFeatureMerger	List Attribute to Single Attribute	No	Yes	Features	Only joining transformer to use all list attribute contents as keys (others may use one specific value in a list, but not the entire list)	Merges the attributes and/or geometry of one set of features onto another set of features, based on matching list attribute values with key attribute values and expressions.
InlineQuerier	SQL query	Yes	No	Features	Use SQL on non-SQL data Allows multiple join/SQL statements, and so can replace multiple transformers with one	Creates a set of SQLite database tables from incoming features, executes SQL queries against them, and outputs the results as features.
SQLCreator	SQL query	Yes	No	External DB	Best if all features already in SQL-capable source Read an external DB table mid-translation	Generates FME features from the results of a SQL query executed once against a database. One FME feature is created for each row of the results of the SQL query.
SQLExecutor	SQL query	Yes	No	External DB	Best for complex joins between features and SQL-capable source Read an external DB table mid-translation	Executes SQL queries against a database. One query is issued to the database for each initiating feature that enters the transformer. Both the initiating features and the results of the query may be output as features.
DatabaseJoiner	Attributes	No	Yes	External DB and Features	Best for simple joins between features and a database table Very efficient if keys indexed, and caches data locally Read an external DB table mid-translation Non-blocking	Joins attributes from an external table to features already in a workspace, based on a common key or keys. SQL knowledge not required. Non-blocking transformer.
Matcher	Geometry and/or Attributes	No	Yes	Features	The only one of these methods that can use geometry as key for matching Can look for attribute differences Uses geometry, not spatial relationships	Detects features that are matches of each other. Features are declared to match when they have matching geometry, matching attribute values, or both. A list of attributes which must differ between the features may also be specified. If matching on attributes only (not geometry), using the FeatureMerger or another method will give better performance.

Configuration

Input Ports

Output Ports

Parameters

Group Processing

Group By

The input features may be partitioned by the Group By parameter. If you choose any Group By attributes, then references between features will only be resolved if they share a common value for the selected attributes.

If you do not choose any Group By attributes, all features are processed together.

If you have more than one Reader, a typical use is to group by reader_id to ensure that references are resolved within the correct set of features.

Complete Groups

Select the point in processing at which groups are processed:

When All Features Received	This is the default behavior. Processing will only occur in this transformer once all input is present.
When Group Changes (Advanced)	This transformer will process input groups in order. Changes of the value of the Group By parameter on the input stream will trigger processing on the currently accumulating group. This may improve overall speed (particularly with multiple, equally-sized groups), but could cause undesired behavior if input groups are not truly ordered.

Join Attributes

Requestor List Attribute	The list attribute on Requestor features whose keys will be matched against the Supplier Attribute on Supplier features.
Supplier Attribute	The attribute on Supplier features that will be matched against the keys in the Requestor List Attribute for Requestor features.
Comparison Mode	Specifies how to perform the comparison between Requestor and Supplier attribute values. If Automatic or Numeric is specified, an attempt will be made to convert attribute values to numbers before comparing them. If the numeric conversion fails, string comparison will be used.

Merge Parameters

Feature Merge Type	Attributes Only: Supplier attributes will be joined to Requestor features. Attributes will be merged according to the Attribute Accumulation and Generate List parameters Geometry: Supplier geometry will be merged to the Requestors, and will replace any existing geometry. Attributes are not merged. Attributes and Geometry: Both the geometry and attributes from the suppliers are joined to the Requestor features. Any geometry on the Requestor will be overwritten. Attributes will be merged according to the Attribute Accumulation and Generate List parameters.
Process Duplicate Suppliers	If more than one Supplier is found for a given Requestor, and Process Duplicate Suppliers is No, then every Supplier after the first is output via the <Rejected> port and only the first of the Suppliers will be matched with a Requestor. If set to Yes, then the duplicate Suppliers are all matched with the corresponding Requestor merging attributes based on the Attribute Accumulation mode, and optionally output as an attribute list of the Requestor, using the specified List Name. The Supplier geometry is merged using the specified Geometry Merge Type.
Geometry Merge Type	Build Polygons: If the Suppliers consist exclusively of polygon and donut polygon features, any common border segments will be removed. If the Suppliers contain at least one non-donut or non-polygon feature, then the transformer will form polygons and donuts from the Suppliers and will join connected line segments of the Supplier features before setting the geometry of the Requestor feature. In this case, the geometry may be an aggregate if several disjoint geometries were created. Build Aggregates: The transformer will create an aggregate of the geometries of the Supplier features. (If there is only one Supplier feature, then the Requestor geometry will be an aggregate with one element.) Build Lines from Points: The transformer will connect the points of the Supplier features into lines. Note that any non-point features that are referenced will be ignored when building lines.
Incomplete Requestors	This parameter governs what happens to partially matched Requestors. If this parameter is set to Merge Supplier Information, then the Suppliers that were found will be merged onto the Requestor and then output via the UnmergedRequestor port. The Suppliers used will be output via the UsedSuppliers port. If this parameter is set to Do Not Merge Supplier Information, then the Requestor will be output untouched via the UnmergedRequestor port and the Suppliers will be output via the UnusedSupplier port.

Attribute Accumulation

If attributes on the Supplier and Requestor feature share the same name, but are not geometry attributes that start with fme_, then they are deemed conflicted.

Accumulation Mode	Merge Supplier: The Requestor feature will retain all of its own un-conflicted attributes, and will additionally acquire any un-conflicted attributes that the Supplier feature has. This mode will handle conflicted attributes based on the Conflict Resolution parameter. Prefix Supplier: The Requestor feature will retain all of its own attributes. In addition, the Requestor will acquire attributes reflecting the Supplier feature’s attributes, with the name prefixed with the Prefix parameter. Replace with Single Supplier: The Requestor feature will have all of its attributes removed, except geometry attributes that start with fme_. Then, all of the attributes from one (arbitrary) Supplier feature will be placed onto the Requestor.
Conflict Resolution	Use Requestor: If a conflict occurs, the Requestor values will be maintained. Use Supplier: If a conflict occurs, the values of the Supplier will be transferred onto the Requestor.
Ignore Nulls	No: Treat null attribute values like other attribute values. Yes: Treat null attribute values as less important than other attribute values. Whenever a null value is merged with a non-null value, the non-null value shall prevail, regardless of what Conflict Resolution is set to. Note: Null and other attribute values are always more dominant than missing attribute values. That is, whenever an attribute value is merged with a missing attribute value, the attribute value shall prevail, regardless of the Ignore Nulls and Conflict Resolution settings.
Prefix	To prevent a Supplier attribute from being ignored because the Requestor attribute already exists, you can optionally specify a prefix that will be applied to each Supplier attribute when it is added to the Requestor. When there are multiple Supplier features for a Requestor feature, multiple Supplier attribute values are merged into the same prefixed attribute. When multiple Supplier features have the same attribute, generally the resulting attribute value is taken from the last of these features. However, this process is governed by the Ignore Nulls parameter.

Generate List

If there are duplicate Suppliers and Generate List is enabled, then any Suppliers that are combined with a Requestor will have their attributes added to the specified list on the Requestor.

Note: List attributes are not accessible from the output schema in FME Workbench unless they are first processed using a transformer that operates on them, such as ListExploder or ListConcatenator. Alternatively, AttributeExposer can be used.

List Name

Enter a name for the list attribute.

Add To List

All Attributes: Every attribute from a Supplier that combined with a Requestor will be added to the list specified in List Name.

Selected Attributes: Only the attributes specified in the Selected Attributes parameter will be added to the list specified in List Name.

Selected Attributes

The attributes to be added to the list when Add To List is Selected Attributes.

Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

How to Set Parameter Values

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

Content Types

String Functions	These functions manipulate and format strings.
Special Characters	A set of control characters is available in the Text Editor.
Math Functions	Math functions are available in both editors.
Date/Time Functions	Date and time functions are available in the Text Editor.
Math Operators	These operators are available in the Arithmetic Editor.
FME Feature Functions	These return primarily feature-specific values.
FME Parameters	FME and workspace-specific parameters may be used.
Creating and Modifying User Parameters	Create your own editable parameters.

Dialog Options - Tables

Transformers with table-style parameters have additional tools for populating and manipulating values.

Table Tools

Row Reordering

Enabled once you have clicked on a row item. Choices include:

Add a row
Remove a row
Move current row up one
Move current row down one
Move current row to top
Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

Cut a row - delete and copy to clipboard
Copy a row to the clipboard
Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

Reference

Processing Behavior	Group-Based
Feature Holding	Yes
Dependencies	None
Aliases
History

FME Community

The FME Community is the place for demos, how-tos, articles, FAQs, and more. Get answers to your questions, learn from other users, and suggest, vote, and comment on new features.

Search for all results about the ListBasedFeatureMerger on the FME Community.

Examples may contain information licensed under the Open Government Licence – Vancouver and/or the Open Government Licence – Canada.