QGIS/resources/function_help/json/sqlite_fetch_and_increment

{
  "name": "sqlite_fetch_and_increment",
  "type": "function",
  "groups": ["Record and Attributes"],
  "description": "Manage autoincrementing values in sqlite databases.<p>SQlite default values can only be applied on insert and not prefetched.</p><p>This makes it impossible to acquire an incremented primary key via AUTO_INCREMENT before creating the row in the database. Sidenote: with postgres, this works via the option <i>evaluate default values</i>.</p><p>When adding new features with relations, it is really nice to be able to already add children for a parent, while the parents form is still open and hence the parent feature uncommitted.</p><p>To get around this limitation, this function can be used to manage sequence values in a separate table on sqlite based formats like gpkg.</p><p>The sequence table will be filtered for a sequence id (filter_attribute and filter_value) and the current value of the id_field will be incremented by 1 and the incremented value returned.</p><p>If additional columns require values to be specified, the default_values map can be used for this purpose.</p><p><b>Note</b><br/>This function modifies the target sqlite table. It is intended for usage with default value configurations for attributes.</p><p>When the database parameter is a layer and the layer is in transaction mode, the value will only be retrieved once during the lifetime of a transaction and cached and incremented. This makes it unsafe to work on the same database from several processes in parallel.</p>",
  "arguments": [
	  {"arg":"database", "description":"Path to the sqlite file or geopackage layer"},
    {"arg":"table", "description":"Name of the table that manages the sequences"},
    {"arg":"id_field", "description":"Name of the field that contains the current value"},
    {"arg":"filter_attribute", "description":"Name the field that contains a unique identifier for this sequence. Must have a UNIQUE index."},
    {"arg":"filter_value", "description":"Name of the sequence to use."},
    {"arg":"default_values", "description":"Map with default values for additional columns on the table. The values need to be fully quoted. Functions are allowed.", "optional": true}
  ],
  "examples": [
    { "expression":"sqlite_fetch_and_increment(@layer, 'sequence_table', 'last_unique_id', 'sequence_id', 'global', map('last_change', 'date(''now'')', 'user', '''' || @user_account_name || ''''))", "returns":"0"},
    { "expression":"sqlite_fetch_and_increment(layer_property(@layer, 'path'), 'sequence_table', 'last_unique_id', 'sequence_id', 'global', map('last_change', 'date(''now'')', 'user', '''' || @user_account_name || ''''))", "returns":"0"}
  ]
}