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This extension allows a GeoPackage to contain additional data that is related to geospatial (e.g., features) or attributes data. As an example, this can be used to establish a many-to-many relationship between features (e.g., points, lines, or areas) and multimedia files. By definition, the “left” side of the relationship is the “base” data and the “right” side of the relationship is the “related” data.
gpkg_extensionsTo use this extension, add the following rows to this table.
| table_name | column_name | extension_name | definition | scope |
|---|---|---|---|---|
gpkgext_relations |
null | gpkg_related_tables |
http://docs.opengeospatial.org/is/18-000/18-000.html | read-write |
| name of actual User Defined Mapping Table | null | gpkg_related_tables |
http://docs.opengeospatial.org/is/18-000/18-000.html | read-write |
gpkgext_relationsThis table describes extended relationships. It requires the following columns:
| Column | Description |
|---|---|
id |
primary key |
base_table_name |
Name of the table containing the base data (e.g., features) to relate |
base_primary_column |
Name of the primary key column in base_table_name |
related_table_name |
Name of the table containing the related content |
related_primary_column |
Name of the primary key column in related_table_name |
relation_name |
Name (profile) of the relationship |
mapping_table_name |
Name of a user-defined mapping table |
Add a row to this table for each relationship.
Hint: To further define the semantics for a relationship, consider using the Schema Extension on the
mapping_table_namecolumn.
A user-defined mapping table describes the many-to-many relationships between base data and related data. A user-defined mapping table requires at least the following columns:
| Column | Description |
|---|---|
base_id |
The primary key value of the base data table |
related_id |
The primary key value of the related data table |
Add a row to this table for each related pair.
This extension offers a number of profiles. Each profile adds some specialized rules to make relationships more meaningful.
When the relation_name of a relationship is “media”, the related table must be an attributes table with at least the following columns:
| Column | Description |
|---|---|
| any | Primary Key |
data |
BLOB containing multimedia content |
content_type |
MIME type of data |
Hint: If you want to avoid data duplication, add a column to the user-defined mapping table containing a hash of the data. That way you can check to see whether a piece of media already exists in the table before adding it.
When the relation_name of a relationship is “simple_attributes”, the related table must be an attributes table and that table must have TEXT, INTEGER, or REAL columns (no BLOB or NULL).
When the relation_name of a relationship is “features”, the related table must be a features table.