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How can I automatically attach/create fields to an entity when I enable a module that provides entities?
For example, I want my Entity to have an Image field, and a handful of text, date, and number fields ready to roll.
I see field_create_field and field_create_instance, do I need to define them or can I use whats already defined as fields in Drupal and just give them labels and default values?
field_create_field creates a field definition, and field_create_instance creates an instance of that field.
If you look at the UI, you are able to reuse the same field different places, this is what field_create_instance does.
Both functions requires an array with a lot of data containing info about which entity to attach the field on, labels various settings, how the field should be displayed etc.
This is a just a follow up to @googletorp's answer, you can use the following code to export an existing field instance definition to PHP code which you can then alter as you need to for your new instance:
$entity_type = 'node';
$field_name = 'field_name';
$bundle_name = 'node_type';
$info_config = field_info_field($field_name);
$info_instance = field_info_instance($entity_type, $field_name, $bundle_name);
unset($info_config['id']);
unset($info_instance['id'], $info_instance['field_id']);
include_once DRUPAL_ROOT . '/includes/utility.inc';
$output = "field_create_field(" . drupal_var_export($info_config) . ");\n";
$output .= "field_create_instance(" . drupal_var_export($info_instance) . ");";
drupal_set_message("<textarea rows=30 style=\"width: 100%;\">". $output .'</textarea>');
That will give you both the field_create_field and field_create_instance code for the field.
field_create_instance(... part of the field code as well so it's a straight copy and paste
I always solve this by creating the appropriate entity configuration in an install, exporting it with Features, and then make the install profile depend on it. As googletorp points out, manually creating a field requires a lot of data, and your bound to mess it up quite a few times if you write it all manually.
Even if you don't want to use Features for storing the field, you could still export them that way, and then copy paste the code into your own module.
