Create validation constraints on nodes to entity validation

* The node changed date must not be lower than the changed date in the DB, otherwise the form submission gets rejected. Do we want to have that as generic constraint?

Sounds reasonable!

* The title is required if the content type supports the title property. How do we implement that as validation constraint?

We can conditionally add in the constraint in the title's getConstraint() method.

+++ b/core/modules/node/lib/Drupal/node/NodeStorageController.php
@@ -155,6 +155,11 @@ public function baseFieldDefinitions() {
+      'constraints' => array(
+        'ComplexData' => array(

this should use property_constraints

+++ b/core/modules/node/lib/Drupal/node/Field/TitleField.php
@@ -0,0 +1,32 @@
+ * Defines the 'node_title' field with an extra constraint.

Maybe call it a custom constraint, because something extra can live in the field definition also?
Or a field generating custom constraints.

+++ b/core/modules/node/lib/Drupal/node/Plugin/Validation/Constraint/NodeChangedConstraint.php
@@ -0,0 +1,25 @@
+ * Supports validating all primitive types.
+ *
+ * @Plugin(
+ *   id = "NodeChanged",

Wrong docs.

+++ b/core/modules/node/lib/Drupal/node/Tests/NodeValidationTest.php
@@ -0,0 +1,77 @@
+    $violation = $violations->get(0);

You can use array access here.

+++ b/core/modules/node/lib/Drupal/node/Field/TitleField.php
@@ -0,0 +1,32 @@
+ * Defines the 'node_title' field with an extra constraint.

Maybe call it a custom constraint, because something extra can live in the field definition also?
Or a field generating custom constraints.

+++ b/core/modules/node/lib/Drupal/node/Plugin/Validation/Constraint/NodeChangedConstraint.php
@@ -0,0 +1,25 @@
+ * Supports validating all primitive types.
+ *
+ * @Plugin(
+ *   id = "NodeChanged",

Wrong docs.

+++ b/core/modules/node/lib/Drupal/node/Tests/NodeValidationTest.php
@@ -0,0 +1,77 @@
+    $this->assertEqual(count($violations), 1, 'Violation found when title is too long.');
+    $violation = $violations->get(0);
...
+    $violation = $violations->get(0);

You can use array access here.

+++ b/core/modules/node/lib/Drupal/node/Field/TitleField.php
@@ -0,0 +1,32 @@
+    // If the node type is configured to have a title field then it should be
+    // required.

What is saved as node title if the title is not required?
I'd expect 'required' => TRUE to be on the node title usually?

Looks like we could really need a way to customize/override field definitions by bundle... :/

+++ b/core/modules/node/lib/Drupal/node/Tests/NodeValidationTest.php
@@ -0,0 +1,77 @@
+    $this->assertEqual(count($violations), 1, 'Violation found when title is too long.');
+    $violation = $violations->get(0);

On the issue introducing validation we agreed on checking the violation message also - so we are sure the right constraint failed.

$this->assertEqual($violations->count(), 1, 'Validation failed.');
$this->assertEqual($violations[0]->getMessage(), t('This value should not be null.'));

The node title is not required per se, so we cannot specify that on the entity base properties.

Yes. But I'm wondering whether it isn't still required. Looking at the schema I think it technically is required:

      'title' => array(
        'description' => 'The title of this node, always treated as non-markup plain text.',
        'type' => 'varchar',
        'length' => 255,
        'not null' => TRUE,
        'default' => '',
      ),

So let's follow that? Make it required + provide a default value of '' i.e. you may not unset it. Then the node type setting influences the form behaviour, while at storage level a titel is required?

Without that, I think you could unset it and trigger storage exceptions.

+++ b/core/modules/node/lib/Drupal/node/Field/TitleField.php
@@ -0,0 +1,32 @@
+ * @file
+ * Contains \Drupal\node\Plugin\DataType\TitleField.

I realized this is in the wrong namespace - it's not a data type plugin.

+++ b/core/modules/node/lib/Drupal/node/Field/TitleField.php
@@ -18,7 +18,9 @@ class TitleField extends Field {
-    $constraints = parent::getConstraints();
+    // Don't call parent::getConstraints() here, we have our own handling for
+    // required title fields.
+    $constraints = array();

So why do we skip required validation here then? With '' it should pass?

Yes, exactly!