Understanding Drupal Core Interfaces: Field API

The Drupal Field API is the foundational system used to define, manage, and store structured data on entities like content nodes, users, and taxonomy terms. It allows developers to extend the base content model by attaching custom fields programmatically or via the Field UI module.

Core Components

Every field in Drupal is composed of three primary plugin types:

Field Types: Define the data structure and storage schema (e.g., text, integer, image).
Field Widgets: Manage the form elements used for data entry in the edit interface.
Field Formatters: Control how the field data is rendered and displayed to end users.

Primary Data Structures

The API distinguishes between how data is defined and where it is applied:

FieldStorage: Defines a particular data type (e.g., an "Image" storage definition) that can be shared across multiple bundles.
Field (Instance): The specific attachment of a storage definition to a Bundle (like an "Article" content type), containing bundle-specific settings.
Bundles: Groups of fields treated as a single unit for a specific entity type.

Developer APIs

Field Types API: Used to create new, custom field types by implementing FieldItemInterface.
Field Attach API: Connects entities to the Field API, handling loading, saving, and generating form structures for fieldable entities.
Field Storage API: Provides the backend logic for database storage, defaulting to SQL tables for each field.

Proactive Follow-up: Are you looking to create a custom field type programmatically, or do you need help querying field data for an existing entity?

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To implement a custom field type and query field data, follow these standard Drupal practices.

1. Create a Custom Field Type

Custom field types are defined as plugins. Create a PHP class in your module's src/Plugin/Field/FieldType directory.

Define the Plugin: Use the #[FieldType] attribute to declare your field's unique ID, label, and default widget/formatter.
Implement Core Methods: Your class must extend FieldItemBase and implement these key methods:
- schema(): Defines the database columns (using the Schema API).
- propertyDefinitions(): Describes the meta-data of your field's properties to the Typed Data API.
- isEmpty(): Determines if a value exists to avoid saving blank records.

2. Query Field Data

To retrieve entities based on field values, use the Entity Query API.

Initialize the Query: Use \Drupal::entityQuery('entity_type_id') or the entityTypeManager service.
Add Conditions: Filter by your custom field's machine name and value.
Execute and Load: The query returns entity IDs; you must then load the entities to access their full data.

php

// Example: Querying nodes where your custom field equals 'value'
$ids = \Drupal::entityQuery('node')
  ->condition('field_custom_name', 'target_value')
  ->accessCheck(TRUE) // Mandatory in newer Drupal versions
  ->execute();

$nodes = \Drupal::entityTypeManager()->getStorage('node')->loadMultiple($ids);

Use code with caution.

Actionable Tools

Development: Use the Examples for Developers module for boilerplate code of field types.
UI Testing: After creating your field type, enable your module and visit Structure > Content types > Manage fields to verify it appears in the list.
Debugging: Use Devel's kint() or dump() to inspect entity objects and field structures during development.

These articles provide guidance on creating custom Drupal field types and querying field data:

Proactive Follow-up: Would you like a starter code snippet for a specific field property (like a text string or decimal), or do you need the code for the Widget and Formatter plugins as well?

To build a custom field in Drupal, you must implement three specific Plugin types. In modern Drupal (10/11), these use PHP Attributes for discovery.

1. Field Type (FieldItemBase)

This defines the data structure in the database.
File: src/Plugin/Field/FieldType/RealnameItem.php

Here are starter code examples for the three main components needed to create a custom field in Drupal: the Field Type, Field Widget, and Field Formatter.

The Field Type defines the data structure stored in the database. A common example is storing first and last names. Drupalize.Me

The Field Widget component handles the form elements displayed when a user is editing content that uses your custom field. This could involve simple text fields for first and last names. Drupalize.Me

The Field Formatter determines how the field's data is presented to visitors on the website. For instance, it might combine the first and last names into a single "Full Name" display. Drupalize.Me

For these components to function correctly in your module:

Use PHP Attributes (#[FieldType], #[FieldWidget], #[FieldFormatter]) for modern Drupal versions. Drupal.org
Ensure the Namespace in your plugin files matches your module's folder structure to enable Autoloading.
Always Clear Cache after adding new plugin files for them to be recognized.