How do I fetch the values of an entity's field?

Question

How to query all the values of a particular field. I used Drupal\Core\Database\Connection object to fetch the values as below:

$connection = \Drupal::database();
$result = $connection->select('node__body', 'n')
  ->fields('n', array('body_value'))
  ->execute()->fetchCol();

Is there any Drupal 8 API specifically for this? I need to fetch all values of the field and fetch the field values for a particular node bundle.

Answer 1

In Drupal 7, you would use the EntityFieldQuery class. In Drupal 8, there is an equivalent service.

$query = \Drupal::entityQuery('node')
    ->condition('status', 1)
    ->condition('changed', REQUEST_TIME, '<')
    ->condition('title', 'cat', 'CONTAINS')
    ->condition('field_tags.entity.name', 'cats');

$nids = $query->execute();

In your case, instead of field_tag.entity.name, you need body.body_value. See EntityFieldQuery has been rewritten.
If you just need to get all the node IDs, remove all the condition() lines, and then use entity_load_multiple('node', $nids) to load the full node objects.

In this way, your code is not making assumptions about the tables used to contain the node data. Plus, it can be easily extended to filter the nodes basing on some criteria (node properties, content of fields...).

Answer 2

Although entityQuery is very powerful I believe not everything can and should be done with it. Loading all nodes to get the values might consume to much resources. Using the Database API should be fine with some help of the TableMappingInterface, as stated in the documentation:

Provides a common interface for mapping field columns to SQL tables.

Warning: using methods provided here should be done only when writing code that is explicitly targeting a SQL-based entity storage. Typically this API is used by SQL storage classes, or other SQL-specific code like the Views integration code for the Entity SQL storage. Another example of legal usage of this API is when needing to write a query that \Drupal::entityQuery() does not support.

/** @var \Drupal\Core\Entity\Sql\TableMappingInterface $table_mapping*/
$table_mapping = \Drupal::entityTypeManager()->getStorage('node')->getTableMapping();

$field_table = $table_mapping->getFieldTableName('field_categories');
$field_storage_definitions = \Drupal::service('entity_field.manager')->getFieldStorageDefinitions('node')['field_categories'];
$field_column = $table_mapping->getFieldColumnName($field_storage_definitions, 'target_id');

$connection = \Drupal::database();
$result = $connection->select($field_table, 'f')
  ->fields('f', array($field_column))
  ->distinct(TRUE)
  ->condition('bundle', 'article')
  ->condition('entity_id', $nids, 'IN')
  ->execute()->fetchCol();

This works so far. But there are still asumptions about the columns bundle and entity_id. I am not sure if it would be save to assume that they will be allways there. Maybe there are also functions to get the bundle and the entity-id column, but I guess not, because it seems that views makes the same asumptions, see views_field_default_views_data.