Added syntactic sugar for extending mappings

README.md

@@ -19,6 +19,7 @@ Transfers data from one object to another, allowing custom mapping operations.
         * [ReverseMap](#reversemap)
         * [Copying a mapping](#copying-a-mapping)
     * [Automatic creation of mappings](#automatic-creation-of-mappings)
+    * [Extending mappings](#extending-mappings)
     * [Resolving property names](#resolving-property-names)
         * [Naming conventions](#naming-conventions)
         * [Explicitly state source property](#explicitly-state-source-property)
@@ -442,6 +443,21 @@ $config->getOptions()->createUnregisteredMappings();
 With this configuration the mapper will generate a very basic mapping on the
 fly instead of throwing an exception if the mapping is not configured.
 
+### Extending mappings
+There are some cases when you want to use unregistered mappings but you want to setup 
+mapping just for one or few properties of the mapped class without need to define mapping for 
+each property. In this case you can simply extend mapping and define just those properties
+that you need to have mapped differently:
+
+```php
+<?php
+
+$config->extendMapping(Employee::class, EmployeeListView::class)
+    ->forMember('name', function (Employee $employee) {
+        return strtoupper($employee->name);
+    });
+```
+
 ### Resolving property names
 Unless you define a specific way to fetch a value (e.g. `mapFrom`), the mapper
 has to have a way to know which source property to map from. By default, it will

src/Configuration/AutoMapperConfig.php

@@ -202,6 +202,22 @@ public function registerMapping(
         return $mapping;
     }
 
+    /**
+     * @inheritdoc
+     */
+    public function extendMapping(
+        string $sourceClassName,
+        string $destinationClassName
+    ): MappingInterface {
+        if (!$this->hasMappingFor($sourceClassName, $destinationClassName)) {
+            throw new \RuntimeException("Undefined mapping for class $sourceClassName");
+        }
+
+        $mapping = $this->getMappingFor($sourceClassName, $destinationClassName);
+
+        return $this->registerMapping($sourceClassName, $destinationClassName)->copyFromMapping($mapping);
+    }
+
     /**
      * @inheritdoc
      */

src/Configuration/AutoMapperConfigInterface.php

@@ -47,6 +47,19 @@ public function registerMapping(
         string $destinationClassName
     ): MappingInterface;
 
+    /**
+     * Extends already registered mapping. Whether it's unregistered type
+     * of mapping or manually registered mapping.
+     *
+     * @param string $sourceClassName
+     * @param string $destinationClassName
+     * @return MappingInterface
+     */
+    public function extendMapping(
+        string $sourceClassName,
+        string $destinationClassName
+    ): MappingInterface;
+
     /**
      * @return Options
      */

test/AutoMapperTest.php

@@ -150,6 +150,40 @@ public function testItCanMapWithACallback()
         $this->assertEquals('NewName', $destination->name);
     }
 
+    public function testItCanExtendMapping()
+    {
+        $this->config->registerMapping(Source::class, Destination::class);
+        $mapper = new AutoMapper($this->config);
+        $source = new Source();
+        $source->name = 'John';
+        $destination = $mapper->map($source, Destination::class);
+
+        $this->assertEquals('John', $destination->name);
+
+        $this->config->extendMapping(Source::class, Destination::class)
+            ->forMember('name', function (Source $source) {
+                return $source->name . ' ' . 'Doe';
+            });
+        $destination = $mapper->map($source, Destination::class);
+
+        $this->assertEquals('John Doe', $destination->name);
+    }
+
+    public function testItThrowsRuntimeErrorOnExtendMappingWhenDoesntHaveMapping()
+    {
+        $this->expectException(\RuntimeException::class);
+
+        $this->config->getOptions()->dontCreateUnregisteredMappings();
+        $mapper = new AutoMapper($this->config);
+        $this->config->extendMapping(Source::class, Destination::class)
+            ->forMember('name', function (Source $source) {
+                return $source->name . ' ' . 'Doe';
+            });
+        $source = new Source();
+        $source->name = 'John';
+        $mapper->map($source, Destination::class);
+    }
+
     public function testTheConfigurationCanBeRetrieved()
     {
         $config = new AutoMapperConfig();