Add primaryKey() to context classes.

Add a primaryKey method to the context classes. This will let FormHelper
get primaryKey data so that create/update forms can be detected properly
for a given bag of data.

src/View/Form/ArrayContext.php

@@ -33,11 +33,28 @@
  * - `schema` An array of data that emulate the column structures that
  *   Cake\Database\Schema\Table uses. This array allows you to control
  *   the inferred type for fields and allows auto generation of attributes
- *   like maxlength, step and other HTML attributes.
+ *   like maxlength, step and other HTML attributes. If you want
+ *   primary key/id detection to work. Make sure you have provided a `_constraints`
+ *   array that contains `primary`. See below for an exmaple.
  * - `errors` An array of validation errors. Errors should be nested following
  *   the dot separated paths you access your fields with.
- * - `isCreate` A boolean that indicates whether or not this form should
- *   be treated as a create operation. If false this form will be an update.
+ *
+ *  ### Example
+ *
+ *  {{{
+ *  $data = [
+ *    'schema' => [
+ *      'id' => ['type' => 'integer'],
+ *      'title' => ['type' => 'string', 'length' => 255],
+ *      '_constraints' => [
+ *        'primary' => ['type' => 'primary', 'columns' => ['id']]
+ *      ]
+ *    ],
+ *    'defaults' => [
+ *      'id' => 1,
+ *      'title' => 'First post!',
+ *    ]
+ *  ];
  */
 class ArrayContext {
 
@@ -68,18 +85,47 @@ public function __construct(Request $request, array $context) {
 			'required' => [],
 			'defaults' => [],
 			'errors' => [],
-			'create' => true,
 		];
 		$this->_context = $context;
 	}
 
+/**
+ * Get the fields used in the context as a primary key.
+ *
+ * @return array
+ */
+	public function primaryKey() {
+		if (
+			empty($this->_context['schema']['_constraints']) ||
+			!is_array($this->_context['schema']['_constraints'])
+		) {
+			return [];
+		}
+		foreach ($this->_context['schema']['_constraints'] as $data) {
+			if (isset($data['type']) && $data['type'] === 'primary') {
+				return isset($data['columns']) ? (array)$data['columns'] : [];
+			}
+		}
+		return [];
+	}
+
 /**
  * Returns whether or not this form is for a create operation.
  *
+ * For this method to return true, both the primary key constraint
+ * must be defined in the 'schema' data, and the 'defaults' data must
+ * contain a value for all fields in the key.
+ *
  * @return boolean
  */
 	public function isCreate() {
-		return (bool)$this->_context['create'];
+		$primary = $this->primaryKey();
+		foreach ($primary as $column) {
+			if (!isset($this->_context['defaults'][$column])) {
+				return false;
+			}
+		}
+		return true;
 	}
 
 /**

src/View/Form/EntityContext.php

@@ -129,6 +129,19 @@ protected function _prepare() {
 		$this->_tables[$alias] = $table;
 	}
 
+/**
+ * Get the primary key data for the context.
+ *
+ * Gets the primary key columns from the root entity's schema.
+ *
+ * @return boolean
+ */
+	public function primaryKey() {
+		$table = $this->_tables[$this->_rootName];
+		$schema = $table->schema();
+		return $schema->primaryKey();
+	}
+
 /**
  * Check whether or not this form is a create or update.
  *

src/View/Form/NullContext.php

@@ -41,6 +41,15 @@ public function __construct(Request $request, array $context) {
 		$this->_request = $request;
 	}
 
+/**
+ * Get the fields used in the context as a primary key.
+ *
+ * @return array
+ */
+	public function primaryKey() {
+		return [];
+	}
+
 /**
  * Returns whether or not this form is for a create operation.
  *

tests/TestCase/View/Form/ArrayContextTest.php

@@ -33,6 +33,35 @@ public function setUp() {
 		$this->request = new Request();
 	}
 
+/**
+ * Test getting the primary key.
+ *
+ * @return void
+ */
+	public function testPrimaryKey() {
+		$context = new ArrayContext($this->request, []);
+		$this->assertEquals([], $context->primaryKey());
+
+		$context = new ArrayContext($this->request, [
+			'schema' => [
+				'_constraints' => 'mistake',
+			]
+		]);
+		$this->assertEquals([], $context->primaryKey());
+
+		$data = [
+			'schema' => [
+				'_constraints' => [
+					'primary' => ['type' => 'primary', 'columns' => ['id']]
+				]
+			],
+		];
+		$context = new ArrayContext($this->request, $data);
+
+		$expected = ['id'];
+		$this->assertEquals($expected, $context->primaryKey());
+	}
+
 /**
  * Test the isCreate method.
  *
@@ -42,14 +71,18 @@ public function testIsCreate() {
 		$context = new ArrayContext($this->request, []);
 		$this->assertTrue($context->isCreate());
 
-		$context = new ArrayContext($this->request, [
-			'create' => false,
-		]);
+		$data = [
+			'schema' => [
+				'_constraints' => [
+					'primary' => ['type' => 'primary', 'columns' => ['id']]
+				]
+			],
+		];
+		$context = new ArrayContext($this->request, $data);
 		$this->assertFalse($context->isCreate());
 
-		$context = new ArrayContext($this->request, [
-			'create' => true,
-		]);
+		$data['defaults'] = ['id' => 2];
+		$context = new ArrayContext($this->request, $data);
 		$this->assertTrue($context->isCreate());
 	}
 

tests/TestCase/View/Form/EntityContextTest.php

@@ -53,6 +53,19 @@ public function setUp() {
 		$this->request = new Request();
 	}
 
+/**
+ * Test getting primary key data.
+ *
+ * @return void
+ */
+	public function testPrimaryKey() {
+		$row = new Article();
+		$context = new EntityContext($this->request, [
+			'entity' => $row,
+		]);
+		$this->assertEquals(['id'], $context->primaryKey());
+	}
+
 /**
  * Test isCreate on a single entity.
  *