NEW: Adding getTableForField(): Ability to infer the actual DataObject that a particular field lives on (at runtime).

code/Migration.php

@@ -79,6 +79,28 @@ public static function getTableColumns($table) {
         return DB::fieldList($table);
     }
 
+    /**
+     * Allows to you fetch the specific table (and thus DataObject) that a particular field exists on, since they're
+     * merged at runtime. This will actually iterate through the class ancestry in order to determine the table in which
+     * a field actually exists.
+     *
+     * @param	string	$className
+     * @param	string	$field
+     * @return	string
+     */
+    public static function getTableForField($className, $field) {
+        // Let's get our hands dirty on this ancestry filth and reference the database because the private static ::$db isn't reliable (seriously).
+        $ancestors = ClassInfo::ancestry($className, true);
+        foreach($ancestors as $ancestor) {
+            if (DataObject::has_own_table($ancestor)) {
+                if (DB::get_schema()->hasField($ancestor, $field)) return $ancestor;
+            }
+        }
+
+        // Still not found.
+        return '';
+    }
+
     /**
      * Drops columns from a database table.
      * Returns array of columns that were dropped
@@ -98,7 +120,7 @@ public static function dropColumnsFromTable($table, array $columns) {
         }
         return $droppedColumns;
     }
-    
+
     /**
      * Add columns to a database table if they don't exist.
      * Returns array of columns that were added
@@ -123,7 +145,7 @@ public static function addColumnsToTable($table, array $columns) {
         }
         return $addedColumns;
     }
-    
+
     /**
      * Gets the value for a single column in a row from the database by the ID column.
      * Useful when a field has been removed from the class' `$db` property,
@@ -181,7 +203,7 @@ public static function getRowValuesFromTable($table, array $fields, $id) {
         }
         return $values;
     }
-    
+
     /**
      * Sets the values for multiple rows on a database table by the ID column.
      * Useful when fields have been removed from the class' `$db` property,
@@ -230,7 +252,7 @@ public static function setRowValuesOnTable($table, array $values, $id = null, $i
         // Nothing was done.
         return false;
     }
-    
+
     /**
      * Simplifies publishing of an actual page instance (since migrations are run from command line).
      *

tests/MigrateTaskTest.php

@@ -4,6 +4,12 @@ class MigrateTaskTest extends SapphireTest {
 
 	protected static $fixture_file = 'MigrateTaskTest.yml';
 
+    protected $extraDataObjects = [
+        Migration_TestParent::class,
+        Migration_TestChild::class,
+        Migration_TestGrandchild::class,
+    ];
+
 	/**
 	 * @var MigrateTask
 	 */
@@ -30,7 +36,6 @@ public function testLatestBatch() {
 		$this->assertEquals(1, MigrateTask::getLatestBatch());
 	}
 
-
 	public function testTransactionRollback() {
 		// TODO: Ensure changes are rolled back in case there is an exception.
 	}
@@ -55,8 +60,13 @@ public function testTransitionField() {
 		// TODO: Make sure values are properly transformed from old -> new
 	}
 
-}
+	public function testGetTableForField() {
+	    $this->assertEquals(Migration_TestParent::class,     Migration::getTableForField(Migration_TestGrandchild::class, 'ParentField'));
+	    $this->assertEquals(Migration_TestChild::class,      Migration::getTableForField(Migration_TestGrandchild::class, 'ChildField'));
+	    $this->assertEquals(Migration_TestGrandchild::class, Migration::getTableForField(Migration_TestGrandchild::class, 'Grandchild'));
+	}
 
+}
 
 class Migration_UnitTestOnly extends Migration implements TestOnly, MigrationInterface {
 
@@ -83,3 +93,27 @@ protected static function exceptionator() {
 	}
 
 }
+
+class Migration_TestParent extends DataObject implements TestOnly {
+
+    private static $db = [
+        'ParentField' => 'Varchar(255)',
+    ];
+
+}
+
+class Migration_TestChild extends Migration_TestParent implements TestOnly {
+
+    private static $db = [
+        'ChildField' => 'Varchar(255)',
+    ];
+
+}
+
+class Migration_TestGrandchild extends Migration_TestChild implements TestOnly {
+
+    private static $db = [
+        'Grandchild' => 'Varchar(255)',
+    ];
+
+}