Drupal - My view has a null module dependency

dependencies:
  module:
    - null

If this does happen to you then you will have problems importing configuration and in our case the view whose configuration file was affected also raised an error when being saved.

Notice: Undefined index: table in Drupal\views\Plugin\views\relationship\RelationshipPluginBase->calculateDependencies()
(line 179 of core/modules/views/src/Plugin/views/relationship/RelationshipPluginBase.php).

uuid: 12345678-1234-5678-ab9ba-9876d54cba32
langcode: en
status: true
dependencies:
  module:
    - null
    - csv_serialization
    - mycustom_schema
    - rest
    - serialization
    - user
    - views_autocomplete_filters
    - views_data_export
    - views_date_format_sql

+---------------+                     +--------------+
|               |                     |              |
|  Line Item    |                     |  Promotion   |
|               |                     |              |
+-------+-------+                     +-------+------+
        |                                     |
        |                                     |
        |    +-----------------------+        |
        |    |                       |        |
        +--->|   LineItemPromotion   <--------+
             |                       |
             +-----------------------+

Notice: Undefined index: table in Drupal\views\Plugin\views\relationship\RelationshipPluginBase->calculateDependencies()
(line 179 of core/modules/views/src/Plugin/views/relationship/RelationshipPluginBase.php).

But the view works.

Roll up your sleeves

So how do we troubleshoot this and find out what is causing the issues:

1. You get an error when you save the view,
2. The config for the view has null in the name of a dependency and
3. You can't import the configuration

The first thing to check is if the schema of the data model is implemented correctly in the modules install file in

hook_schema()

This is where you define the schema so the tables get created when the module is installed. When investigating this it became apparent that the tables were modified in subsequent and successive updates (adding new fields and indexes etc)

hook_update_N()

As it happens, although the schema was updated by these updates, the module schema itself was not updated - this was not the source of our problems, but would be a problem when the module was installed in the future.

see Updating Database Schema and/or Data in Drupal 8 (www.drupal.org/docs/drupal-apis/update-api/updating-database-schema-andor-data-in-drupal-8)

You should always update the schema with such updates.

Plugin Dependencies

But fixing that does not solve our specific issue. Furthermore, our module is present in the list of dependencies for the view and the view works (although it errors when saved and we can't import the configuration). So we need to see why Views can't work out the dependency and places a null in the config.

To do this we need to understand how Views determines the dependencies and see what the actual source of the error is - maybe not our module after all?

When the configuration for a view is written, Views calculates the dependencies of fields and other things (plugins) used in the view. It does this in RelationshipPluginBase:

web/core/modules/views/src/Plugin/views/relationship/RelationshipPluginBase.php
abstract class RelationshipPluginBase extends HandlerBase {

  .... other stuff removed for readability ....
172:   /**
173:    * {@inheritdoc}
174:    */
175:   public function calculateDependencies() { 
176:     $dependencies = parent::calculateDependencies();
177:     // Add the provider of the relationship's base table to the dependencies.
178:     $table_data = $this->getViewsData()->get($this->definition['base']);
179:     $dependencies['module'][] = $table_data['table']['provider'];
180:     return $dependencies;
181:   }
182:
183:}
184:
185:/**
186: * @}
187: */

This is called for all of the plugins (fields in our case) required by the view and this in turn calls getViewData() to find out which module each plugin comes from, to build up a list of dependencies.

When we come to line 178 we go off to web/core/modules/views/src/ViewsData.php which looks at all modules that implement hook_views_data() to see which is the originator of the plugin.

235:   protected function getData() {
236:     $this->fullyLoaded = TRUE;
237: 
238:     if ($data = $this->cacheGet($this->baseCid)) {
238:       return $data->data;
239:     }
240:     else {
241:       $modules = $this->moduleHandler->getImplementations('views_data');
242:       $data = [];
243:       foreach ($modules as $module) {
244:         $views_data = $this->moduleHandler->invoke($module, 'views_data');
245:         // Set the provider key for each base table.
246:         foreach ($views_data as &$table) {
247:           if (isset($table['table']) && !isset($table['table']['provider'])) {
248:             $table['table']['provider'] = $module;
249:           }
250:         }
251:         $data = NestedArray::mergeDeep($data, $views_data);
252:       }
253:       $this->moduleHandler->alter('views_data', $data);
254: 
255:      $this->processEntityTypes($data);
256: 
257:       // Keep a record with all data.
258:       $this->cacheSet($this->baseCid, $data);
259: 
260:       return $data;
261:     }
262:   }

From line 244 on it gets a list of tables for all modules that implement hook_views_data and looks for the table name from the plugin.

If none is found it returns an empty $data and this causes the error:

Undefined index: table

when the RelationshipPluginBase (line 179) is called resulting in 'null' being returned as a dependency.

So in our case we find that when looking for the module that implements the table (mycustom_schema_line_item_promo) we are returned 'null' since it is not found.

The other tables and more importantly the fields they display in the view results are fine.

So we need to investigate the implementation of the hook_views_data in the custom module.

Declare which tables you own

What we find is that the tables (mycustom_schema_line_item and mycustom_schema_promo) whose fields are in the view (and correctly return the module as the dependency) are defined in the module; as are their relationship with the join table. But, the join table itself is not declared in the hook_views_data() and therefore views can't figure out which module it is dependent on.

The following extracts from this hook implementation show how they are declared:

For brevity I have omitted the declarations of all of the fields for the tables (and other custom tables).

// The line item custom table
  $data['mycustom_schema_line_item']['table']['group'] = t('Custom Line Item');

  $data['mycustom_schema_line_item']['table']['base'] = [
    'title' => t('Custom Line Items'),
    'help' => t('Custom Line Items provided by the Custom module.'),
  ];

  // ID.
  $data['mycustom_schema_line_item']['id'] = [
    'title' => t('Line Item ID'),
    'help' => t('Line item unique identifier.'),
    'field' => [
      'id' => 'numeric',
    ],
    'sort' => [
      'id' => 'standard',
    ],
    'filter' => [
      'id' => 'numeric',
    ],
  ];

  // The field to link in the promo.
  $data['mycustom_schema_line_item']['promo'] = [
    'title' => t('Promotion'),
    'help' => t('The names of the promotions applied to this order.'),
    'relationship' => [
      'base' => 'mycustom_schema_line_item_promo',
      'base field' => 'line_item_id',
      'field' => 'id',
      'id' => 'standard',
      'label' => t('Promotion'),
    ],
  ];


  $data['mycustom_schema_promo']['table']['group'] = t('Custom Promotion');

  $data['mycustom_schema_promo']['name'] = [
    'title' => t('Promotion'),
    'help' => t('The name of the promotion reference.'),
    'field' => [
      'id' => 'standard',
    ],
    'sort' => [
      'id' => 'standard',
    ],
    'filter' => [
      'id' => 'string',
    ],
  ];

  // the join details to link the join table to the promo table
  $data['mycustom_schema_promo']['table']['join'] = [
    'mycustom_line_item_promo' => [
      'left_field' => 'promo_id',
      'field' => 'id',
    ],
  ];

return $data;

So although the relationships are defined here (interestingly in two different ways - but let's not get distracted) there is no definition for the link table (mycustom_line_item_promo) itself.

So when Views calculates the dependencies, it can find the details for the Line item table (mycustom_schema_line_item) and Promotions table (mycustom_schema_promo) and their fields but not for the join table (mycustom_schema_line_item_promo) so can't determine which module it is dependent upon.

We need to add some metadata for that table to the hook_views_data().

 
....  
  
  // in our case all we need to add some table meta data so views can see we 'own' the table
  $data['mycustom_schema_line_item_promo']['table']['group'] = t('Custom Promotion');
  
  $data['mycustom_schema_line_item_promo']['table']['base'] = [
    'title' => t('Custom Line Items - Link to promotions'),
    'help' => t('Custom Line Item promotions provided by the Custom module.'),
  ];
  // why not the fields too? Maybe because we don't want them to be included in views results?
  $data['mycustom_line_item_promo']['id'] = [
    'title' => t('Line Item Promo ID'),
    'help' => t('Line item promo unique identifier.'),
    'field' => [
      'id' => 'numeric',
    ],
    'sort' => [
      'id' => 'standard',
    ],
    'filter' => [
      'id' => 'numeric',
    ],
  ];

Once this is added and caches are flushed (the dependency data are cached in the ViewsData getData() so this is important) the view can be saved without error, the config no longer contains a null dependency (it already had the dependency for our module obtained from the other tables that were declared) and we can import the config.

Hopefully this will point others in the right direction if they face the same issue. Of course it might not be Views at the heart of your issue - if you have faced this issue let me know in the comments.