Why is hook_schema() called twice?

Question

I'm working with entities and, in trying to troubleshoot another issue, I noticed that hook_schema() is called twice each time the module is installed. This happened even with a bare bones test module containing only a hook_schema() definition. My questions are:

• Is it normal for hook_schema() to be called twice on module installation?
• Why would hook_schema() be called twice on installation?

Answer 1

It seems to be normal. Looking at the hook_schema documentation, you see that it is invoked in drupal_get_complete_schema() in includes/bootstrap.inc and in drupal_get_schema_unprocessed() in includes/common.inc. The main difference is that the latter does not invoke hook_schema_alter() after the hook_schema is invoked. The first of those functions seems to be called on practically every page load. The second function is called from drupal_install_schema() during the module install process.

Concluding, looking at the code it seems to me that:

• It is normal
• It happens because during installation, Drupal needs the un-altered schema definitions

Answer 2

Actually, hook_schema() is not just used when a module is installed, and Drupal creates the database tables the module needs. It is also used from other functions or methods that need to know the fields a database table contains. For example, EntityFieldQuery::propertyQuery() and drupal_write_record() use it.

Then, when a module is enabled/installed, Drupal (via module_enable()) invokes hook_schema() twice:

• With a call to drupal_get_schema(NULL, TRUE), which invokes hook_schema() for every enabled module
• With a call to drupal_install_schema($module), which invokes hook_schema() for the modules module_enable() is enabling

  // Refresh the schema to include it.
  drupal_get_schema(NULL, TRUE);
  // Update the theme registry to include it.
  drupal_theme_rebuild();
  // Clear entity cache.
  entity_info_cache_clear();

  // Now install the module if necessary.
  if (drupal_get_installed_schema_version($module, TRUE) == SCHEMA_UNINSTALLED) {
    drupal_install_schema($module);

    // Set the schema version to the number of the last update provided
    // by the module.
    $versions = drupal_get_schema_versions($module);
    $version = $versions ? max($versions) : SCHEMA_INSTALLED;

    // If the module has no current updates, but has some that were
    // previously removed, set the version to the value of
    // hook_update_last_removed().
    if ($last_removed = module_invoke($module, 'update_last_removed')) {
      $version = max($version, $last_removed);
    }
    drupal_set_installed_schema_version($module, $version);
    // Allow the module to perform install tasks.
    module_invoke($module, 'install');
    // Record the fact that it was installed.
    $modules_installed[] = $module;
    watchdog('system', '%module module installed.', array('%module' => $module), WATCHDOG_INFO);
  }