Drupal 8 How to add constraint to filed in a content type as (Unique, Count ... ) !

Sometimes you need to make some fields unique, for instance you need to create all articles with unique title.

For Drupal 7 there is a module Unique field but it's not ported to Drupal 8 yet,  to do so there is two options:

1. If your field created using UI or it's already created by Drupal like (Title, Body ...)
   you should implement hook_entity_bundle_field_info_alter and add another constraint using addConstraint like the following:

   <?php
   
   use Drupal\Core\Entity\EntityTypeInterface;
   /**
    * Implements hook_entity_bundle_field_info_alter().
    */
   function YOURMODULE_entity_bundle_field_info_alter(&$fields, EntityTypeInterface $entity_type, $bundle) {
     if ($entity_type->id() === 'ENTITY_TYPE' && $bundle === 'BUNDLE_NAME') {
       if (isset($fields['FIELD_NAME'])) {
         $fields['FIELD_NAME']->addConstraint('UniqueField');
       }
     }
   }

    

2. If you are creating a custom field programmatically you can addConstraint in your baseFieldDefinitions method like the following:

   <?php
   
   public static function baseFieldDefinitions(EntityTypeInterface $entityType) {
    $fields['FIELD_NAME'] = BaseFieldDefinition::create('string')
      ->setLabel(t('MY UNIQUE FIELD'))
      ->addConstraint('UniqueField');
   
    return $fields;
   }

Heres is the complete list of accepted constraints you can pass to addConstraint:

• AllowedValuesConstraint : Checks for the value being allowed.
• ComplexDataConstraint : Validates properties of complex data structures.
• CountConstraint : Count constraint.
• EmailConstraint : Count constraint.
• IsNullConstraint : Null constraint.
• LengthConstraint : Length constraint.
• NotNullConstraint : NotNull constraint.
• PrimitiveTypeConstraint : Supports validating all primitive types.
• RangeConstraint : Range constraint.
• RegexConstraint : Regex constraint.
• UniqueFieldConstraint : Checks if an entity field has a unique value.
• UuidConstraint : Validates a UUID.

Note : All above constraints code located in core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint folder.

You can do even more better, you can Create custom constraint !