Implement hook_farm_constraint() on behalf of all entity and term reference fields.

modules/farm/farm_constraint/farm_constraint.farm_constraint.inc

@@ -0,0 +1,87 @@
+<?php
+
+/**
+ * @file
+ * Farm constraint hook implementations.
+ */
+
+/**
+ * Implements hook_farm_constraint().
+ */
+function farm_constraint_farm_constraint($type, $id) {
+
+  // This implementation of hook_farm_constraint provides constraints on behalf
+  // of all entity and term reference fields in the system.
+
+  // Define the field types, and entity type that they are used for.
+  $field_types = array(
+    'entityreference' => array(
+      'farm_asset',
+      'log',
+      'user',
+    ),
+    'taxonomy_term_reference' => array(
+      'taxonomy_term',
+    ),
+  );
+
+  // Get information about all field instances.
+  $instances = field_info_field_map();
+
+  // Iterate through the instances.
+  foreach ($instances as $field_name => $instance) {
+
+    // If the field type is not one of the ones we care about, skip it.
+    if (!array_key_exists($instance['type'], $field_types)) {
+      continue;
+    }
+
+    // If the entity type does not match the field type, skip it.
+    if (!in_array($type, $field_types[$instance['type']])) {
+      continue;
+    }
+
+    // Load the field info.
+    $field_info = field_info_field($field_name);
+
+    // If this is an entityreference field, and the entity type does not match
+    // the field target type, skip it.
+    if (($instance['type'] == 'entityreference') && ($type != $field_info['settings']['target_type'])) {
+      continue;
+    }
+
+    // Get the database storage details.
+    $storage_details = $field_info['storage']['details'];
+
+    // This only works with SQL, so skip if that information isn't available.
+    if (empty($storage_details['sql']['FIELD_LOAD_CURRENT'])) {
+      continue;
+    }
+
+    // Iterate through the database tables and column information. There should
+    // only be one of each, so collect information about it.
+    $table = '';
+    $column = '';
+    foreach ($storage_details['sql']['FIELD_LOAD_CURRENT'] as $table_name => $data) {
+      $table = $table_name;
+      foreach ($data as $key => $column_name) {
+        $column = $column_name;
+      }
+    }
+
+    // If a table and column are not found, skip.
+    if (empty($table) || empty($column)) {
+      continue;
+    }
+
+    // Finally, query the table to see if there are any references to this
+    // entity.
+    $references = db_query('SELECT COUNT(entity_id) FROM {' . $table . '} WHERE ' . $column . ' = :id', array(':id' => $id))->fetchField();
+
+    // If references were found, a constraint was detected! Return TRUE.
+    if (!empty($references)) {
+      return TRUE;
+    }
+  }
+}
+

modules/farm/farm_constraint/farm_constraint.info

@@ -2,3 +2,4 @@ name = Farm Constraint
 description = Provides a framework for managing foreign key constraints between farmOS entities.
 core = 7.x
 package = farmOS
+dependencies[] = field