diff --git a/src/Colibri/Core/Entity.php b/src/Colibri/Core/Entity.php
index 7f6999b..2ed1cda 100644
--- a/src/Colibri/Core/Entity.php
+++ b/src/Colibri/Core/Entity.php
@@ -141,7 +141,7 @@ public function setVirtual($offset, $value = null)
    */
   public function hashCode()
   {
-    
+    return sha1(json_encode($this->toArray()));
   }
   
   /**
diff --git a/src/Colibri/Extension/AbstractExtension.php b/src/Colibri/Extension/AbstractExtension.php
new file mode 100644
index 0000000..2f15206
--- /dev/null
+++ b/src/Colibri/Extension/AbstractExtension.php
@@ -0,0 +1,87 @@
+<?php
+
+namespace Colibri\Extension;
+
+use Colibri\Common\Callback;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\EventDispatcher\EventSubscriber;
+use Colibri\Exception\NotFoundException;
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Class AbstractExtension
+ * @package Colibri\Extension
+ */
+abstract class AbstractExtension implements ExtensionInterface, EventSubscriber
+{
+  
+  /**
+   * @var ParametersCollection
+   */
+  protected $configuration;
+  
+  /**
+   * AbstractExtension constructor.
+   * @param ParametersCollection $configuration
+   * @throws NotFoundException
+   */
+  public function __construct(ParametersCollection $configuration)
+  {
+    $extensionConfiguration = $configuration->path(sprintf('extensions.%s', $this->getNameNS()));
+    
+    if (null === $extensionConfiguration) {
+      throw new NotFoundException(sprintf('Extension could not be loaded because configuration for "%s" does not exist',
+        $this->getNameNS()));
+    }
+    
+    $this->setConfiguration($extensionConfiguration);
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function setConfiguration(ParametersCollection $collection)
+  {
+    $this->configuration = $collection;
+    
+    return $this;
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function getConfiguration()
+  {
+    return $this->configuration;
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   * @param callable $callback
+   */
+  protected function resolveEntities(EntityLifecycleEvent $event, callable $callback)
+  {
+    $callback = new Callback($callback);
+    
+    foreach ($this->getConfiguration() as $entityClassName => $extensionConfiguration) {
+      if ($event->getEntity() instanceof $entityClassName) {
+        $entity = $event->getEntity();
+        $callback->call(...[$entity, $extensionConfiguration]);
+      }
+    }
+  }
+  
+  /**
+   * @param $variable
+   * @return bool
+   */
+  protected static function isIterable($variable)
+  {
+    if (version_compare(PHP_VERSION, 7.1) === -1) {
+      return is_array($variable) || ($variable instanceof \Traversable);
+    } else {
+      return is_iterable($variable);
+    }
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/AbstractDataFilter.php b/src/Colibri/Extension/EventSubscriber/AbstractDataFilter.php
new file mode 100644
index 0000000..69231ce
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/AbstractDataFilter.php
@@ -0,0 +1,23 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Colibri\Core\ORMEvents;
+use Colibri\Extension\AbstractExtension;
+
+/**
+ * Class AbstractDataFilter
+ * @package Colibri\Extension\EventSubscriber
+ */
+abstract class AbstractDataFilter extends AbstractExtension
+{
+  
+  /**
+   * @return array
+   */
+  public function getEvents()
+  {
+    return [ORMEvents::beforePersist];
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/DataFilter.php b/src/Colibri/Extension/EventSubscriber/DataFilter.php
new file mode 100644
index 0000000..7978b18
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/DataFilter.php
@@ -0,0 +1,58 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Colibri\Core\Entity\EntityInterface;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\Parameters\ParametersCollection;
+use Colibri\Filters\FilterInterface;
+
+/**
+ * Class DataFilter
+ * @package Colibri\Extension\EventSubscriber
+ */
+class DataFilter extends AbstractDataFilter
+{
+  
+  /**
+   * @inheritDoc
+   */
+  public function getNameNS()
+  {
+    return 'dataFilter';
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   */
+  public function beforePersist(EntityLifecycleEvent $event)
+  {
+    $this->resolveEntities($event, function (EntityInterface $entity, ParametersCollection $parameters) {
+      /** @var ParametersCollection $configuration */
+      foreach ($parameters as $propertyName => $configuration) {
+        if ($entity->hasProperty($propertyName) && $configuration->offsetExists('filters')) {
+          $propertyData = $entity->getByProperty($propertyName);
+          $propertyData = $this->applyFilters($propertyData, $configuration->offsetGet('filters')->toArray());
+          $entity->setByProperty($propertyName, $propertyData);
+        }
+      }
+    });
+  }
+  
+  /**
+   * @param $propertyData
+   * @param array $filterClasses
+   * @return array|int|string
+   */
+  protected function applyFilters(&$propertyData, array $filterClasses)
+  {
+    /** @var FilterInterface $filter */
+    foreach ($filterClasses as $filterClass => $filterClassArguments) {
+      $filter = new $filterClass(...$filterClassArguments);
+      $propertyData = $filter->apply($propertyData);
+    }
+    
+    return $propertyData;
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/ResourceLogger.php b/src/Colibri/Extension/EventSubscriber/ResourceLogger.php
new file mode 100644
index 0000000..d93e99a
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/ResourceLogger.php
@@ -0,0 +1,78 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Colibri\Collection\Collection;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\Core\ORMEvents;
+use Colibri\Extension\AbstractExtension;
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Class ResourceLogger
+ * @package Colibri\Extension\EventSubscriber
+ */
+class ResourceLogger extends AbstractExtension
+{
+  
+  protected $entitiesStates;
+  
+  /**
+   * @inheritDoc
+   */
+  public function __construct(ParametersCollection $configuration)
+  {
+    parent::__construct($configuration);
+    
+    $this->entitiesStates = new Collection();
+  }
+  
+  /**
+   * @inheritdoc
+   */
+  public function getEvents()
+  {
+    return [ORMEvents::beforePersist, ORMEvents::afterPersist];
+  }
+  
+  /**
+   * @inheritdoc
+   */
+  public function getNameNS()
+  {
+    return 'resourceLogger';
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   */
+  public function beforePersist(EntityLifecycleEvent $event)
+  {
+    $entity = $event->getEntity();
+    $states = $this->getEntitiesStates();
+  
+    $states->set($entity->hashCode(), $entity);
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   */
+  public function afterPersist(EntityLifecycleEvent $event)
+  {
+    $entity = $event->getEntity();
+    $states = $this->getEntitiesStates();
+  
+    if (!$states->has($entity->hashCode())) {
+    
+    }
+  }
+  
+  /**
+   * @return Collection
+   */
+  public function getEntitiesStates()
+  {
+    return $this->entitiesStates;
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/ResourceLogger/ResourceModelInterface.php b/src/Colibri/Extension/EventSubscriber/ResourceLogger/ResourceModelInterface.php
new file mode 100644
index 0000000..0d83f71
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/ResourceLogger/ResourceModelInterface.php
@@ -0,0 +1,12 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber\ResourceLogger;
+
+/**
+ * Interface ResourceModelInterface
+ * @package Colibri\Extension\EventSubscriber\ResourceLogger
+ */
+interface ResourceModelInterface
+{
+
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/Sluggable.php b/src/Colibri/Extension/EventSubscriber/Sluggable.php
new file mode 100644
index 0000000..81c77bd
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/Sluggable.php
@@ -0,0 +1,70 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Behat\Transliterator\Transliterator;
+use Colibri\Core\Entity\EntityInterface;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\Core\ORMEvents;
+use Colibri\Extension\AbstractExtension;
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Class Sluggable
+ * @package Colibri\Extension\EventSubscriber
+ */
+class Sluggable extends AbstractExtension
+{
+  
+  /**
+   * @return array
+   */
+  public function getEvents()
+  {
+    return [ORMEvents::beforePersist];
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function getNameNS()
+  {
+    return 'sluggable';
+  }
+  
+  /**
+   * @param EntityInterface $entity
+   * @param ParametersCollection $configuration
+   * @return array
+   */
+  protected function getSlugForEntity(EntityInterface $entity, ParametersCollection $configuration)
+  {
+    $parts = [];
+    
+    foreach ($configuration->offsetGet('properties') as $property) {
+      if ($entity->hasProperty($property)) {
+        $parts[] = Transliterator::urlize(Transliterator::transliterate($entity->getByProperty($property)));
+      }
+    }
+
+    $string = implode($configuration->offsetGet('separator'), $parts);
+    
+    return sprintf('%s%s%s', $configuration->get('prefix'), $string, $configuration->get('suffix'));
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   */
+  public function beforePersist(EntityLifecycleEvent $event)
+  {
+    foreach ($this->getConfiguration() as $entityClassName => $propertyConfiguration) {
+      if ($event->getEntity() instanceof $entityClassName) {
+        $entity = $event->getEntity();
+        foreach ($propertyConfiguration as $propertyName => $configuration) {
+          $entity->setByProperty($propertyName, $this->getSlugForEntity($entity, $configuration));
+        }
+      }
+    }
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/Timestampable.php b/src/Colibri/Extension/EventSubscriber/Timestampable.php
new file mode 100644
index 0000000..9fbeffd
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/Timestampable.php
@@ -0,0 +1,84 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Colibri\Common\DateTime;
+use Colibri\Core\Entity\EntityInterface;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\Core\ORMEvents;
+use Colibri\Core\Repository;
+use Colibri\Extension\AbstractExtension;
+use Colibri\Extension\ExtensionException;
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Class Timestampable
+ * @package Colibri\Extension\EventSubscriber
+ */
+class Timestampable extends AbstractExtension
+{
+  
+  const ON_CREATE = 'create';
+  const ON_UPDATE = 'update';
+  
+  /**
+   * @inheritDoc
+   */
+  public function getEvents()
+  {
+    return [ORMEvents::beforePersist];
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function getNameNS()
+  {
+    return 'timestampable';
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   * @return void
+   */
+  public function beforePersist(EntityLifecycleEvent $event)
+  {
+    foreach ($this->getConfiguration() as $entityClassName => $propertyConfiguration) {
+      if ($event->getEntity() instanceof $entityClassName) {
+        /** @var Repository $repository */
+        $entity = $event->getEntity();
+        $repository = $event->getRepository();
+        foreach ($propertyConfiguration as $propertyName => $configuration) {
+          $this->updateEntity($propertyName, $entity, $repository, $configuration);
+        }
+      }
+    }
+  }
+  
+  /**
+   * @param $property
+   * @param EntityInterface $entity
+   * @param Repository $repository
+   * @param ParametersCollection $configuration
+   * @return null
+   * @throws ExtensionException
+   */
+  private function updateEntity($property, EntityInterface $entity, Repository $repository, ParametersCollection $configuration)
+  {
+    if ($configuration->offsetExists('on') && $onEvent = $configuration->offsetGet('on')) {
+  
+      $onEvent = !static::isIterable($onEvent) ? [$onEvent] : $onEvent;
+
+      foreach ($onEvent as $onEventName) {
+        if (($onEventName === Timestampable::ON_CREATE && $repository->isNewEntity($entity)) || $onEventName === Timestampable::ON_UPDATE) {
+          $entity->setByProperty($property, new DateTime());
+        }
+      }
+      
+      return null;
+    }
+  
+    throw new ExtensionException('Wrong configuration passed to Timestampable extension');
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/EventSubscriber/Versionable.php b/src/Colibri/Extension/EventSubscriber/Versionable.php
new file mode 100644
index 0000000..abcaa05
--- /dev/null
+++ b/src/Colibri/Extension/EventSubscriber/Versionable.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace Colibri\Extension\EventSubscriber;
+
+use Colibri\Core\Entity\EntityInterface;
+use Colibri\Core\Event\EntityLifecycleEvent;
+use Colibri\Core\ORMEvents;
+use Colibri\Extension\AbstractExtension;
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Class Versionable
+ * @package Colibri\Extension\EventSubscriber
+ */
+class Versionable extends AbstractExtension
+{
+  
+  /**
+   * @return array
+   */
+  public function getEvents()
+  {
+    return [ORMEvents::beforePersist];
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function getNameNS()
+  {
+    return 'versionable';
+  }
+  
+  /**
+   * @param EntityLifecycleEvent $event
+   */
+  public function beforePersist(EntityLifecycleEvent $event)
+  {
+    $this->resolveEntities($event, function (EntityInterface $entity, ParametersCollection $parameters) {
+      foreach ($parameters->get('properties') as $propertyName) {
+        $version = $entity->getByProperty($propertyName);
+        $entity->setByProperty($propertyName, $version + 1);
+      }
+    });
+  }
+
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/ExtensionException.php b/src/Colibri/Extension/ExtensionException.php
new file mode 100644
index 0000000..db5becc
--- /dev/null
+++ b/src/Colibri/Extension/ExtensionException.php
@@ -0,0 +1,12 @@
+<?php
+
+namespace Colibri\Extension;
+
+/**
+ * Class ExtensionException
+ * @package Colibri\Extension
+ */
+class ExtensionException extends \Exception
+{
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/ExtensionInterface.php b/src/Colibri/Extension/ExtensionInterface.php
new file mode 100644
index 0000000..624c107
--- /dev/null
+++ b/src/Colibri/Extension/ExtensionInterface.php
@@ -0,0 +1,30 @@
+<?php
+
+namespace Colibri\Extension;
+
+use Colibri\Parameters\ParametersCollection;
+
+/**
+ * Interface ExtensionInterface
+ * @package Colibri\Extension
+ */
+interface ExtensionInterface
+{
+
+  /**
+   * @param ParametersCollection $collection
+   * @return mixed
+   */
+  public function setConfiguration(ParametersCollection $collection);
+  
+  /**
+   * @return ParametersCollection
+   */
+  public function getConfiguration();
+  
+  /**
+   * @return string
+   */
+  public function getNameNS();
+
+}
\ No newline at end of file
diff --git a/src/Colibri/Extension/extenstion.configuration.stub.php b/src/Colibri/Extension/extenstion.configuration.stub.php
new file mode 100644
index 0000000..9175fd0
--- /dev/null
+++ b/src/Colibri/Extension/extenstion.configuration.stub.php
@@ -0,0 +1,54 @@
+<?php
+
+// \stdClass means entity class which should been observable
+
+return
+[
+  'extensions' => [
+    
+    'sluggable' => [
+      \stdClass::class => [
+        'slug' => [
+          'properties' => ['name',],
+          'prefix' => null,
+          'suffix' => null,
+          'separator' => '-',
+        ],
+      ],
+    ],
+    'timestampable' => [
+      \stdClass::class => [
+        'created' => [
+          'on' => [
+            'create',
+          ],
+        ],
+        'modified' => [
+          'on' => [
+            'update',
+            'create',
+          ],
+        ],
+      ],
+    ],
+    'versionable' => [
+      \stdClass::class => [
+        'properties' => [
+          'version',
+        ],
+      ],
+    ],
+    'dataFilter' => [
+      \stdClass::class => [
+        'name' => [
+          'filters' => [
+            \Colibri\Filters\ClearHtmlFilter::class => [[],], // clear all
+          ],
+        ],
+      ],
+    ],
+    'resourceLogger' => [
+      // not implemented yet...
+    ],
+  ]
+];
\ No newline at end of file
diff --git a/src/Colibri/Filters/AbstractFilter.php b/src/Colibri/Filters/AbstractFilter.php
new file mode 100644
index 0000000..6d6754e
--- /dev/null
+++ b/src/Colibri/Filters/AbstractFilter.php
@@ -0,0 +1,10 @@
+<?php
+
+namespace Colibri\Filters;
+
+/**
+ * Class AbstractFilter
+ * @package Colibri\Filters
+ */
+abstract class AbstractFilter implements FilterInterface
+{}
\ No newline at end of file
diff --git a/src/Colibri/Filters/ClearHtmlFilter.php b/src/Colibri/Filters/ClearHtmlFilter.php
new file mode 100644
index 0000000..df6b71f
--- /dev/null
+++ b/src/Colibri/Filters/ClearHtmlFilter.php
@@ -0,0 +1,64 @@
+<?php
+
+namespace Colibri\Filters;
+
+/**
+ * Class ClearHtmlFilter
+ * @package Colibri\Filters
+ */
+class ClearHtmlFilter extends AbstractFilter
+{
+  
+  /**
+   * @var array
+   */
+  protected $allowedTags = [
+    'html', 'body',
+    'b', 'br', 'hr', 'i', 'li', 'ol', 'p', 's',
+    'span', 'table', 'tr', 'td',
+    'u', 'ul'
+  ];
+  
+  /**
+   * ClearHtmlFilter constructor.
+   * @param $allowedTags
+   */
+  public function __construct(array $allowedTags = null)
+  {
+    if (null !== $allowedTags && is_array($allowedTags)) {
+      $this->setAllowedTags($allowedTags);
+    }
+  }
+  
+  /**
+   * @inheritDoc
+   */
+  public function apply($input)
+  {
+    $allowedTags = array_map(function ($allowedTag) {
+      return sprintf('<%s>', $allowedTag);
+    }, $this->getAllowedTags());
+
+    return strip_tags($input, implode($allowedTags));
+  }
+  
+  /**
+   * @return array
+   */
+  public function getAllowedTags(): array
+  {
+    return $this->allowedTags;
+  }
+  
+  /**
+   * @param array $allowedTags
+   * @return $this
+   */
+  public function setAllowedTags(array $allowedTags)
+  {
+    $this->allowedTags = $allowedTags;
+    
+    return $this;
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Filters/FilterInterface.php b/src/Colibri/Filters/FilterInterface.php
new file mode 100644
index 0000000..61f965a
--- /dev/null
+++ b/src/Colibri/Filters/FilterInterface.php
@@ -0,0 +1,18 @@
+<?php
+
+namespace Colibri\Filters;
+
+/**
+ * Interface FilterInterface
+ * @package Colibri\Filters
+ */
+interface FilterInterface
+{
+  
+  /**
+   * @param string|integer|array $input
+   * @return string|integer|array
+   */
+  public function apply($input);
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Filters/IntegerFilter.php b/src/Colibri/Filters/IntegerFilter.php
new file mode 100644
index 0000000..f7b82eb
--- /dev/null
+++ b/src/Colibri/Filters/IntegerFilter.php
@@ -0,0 +1,20 @@
+<?php
+
+namespace Colibri\Filters;
+
+/**
+ * Class IntegerFilter
+ * @package Colibri\Filters
+ */
+class IntegerFilter extends AbstractFilter
+{
+  
+  /**
+   * @inheritdoc
+   */
+  public function apply($input)
+  {
+    return (integer)$input;
+  }
+  
+}
\ No newline at end of file
diff --git a/src/Colibri/Query/Criteria.php b/src/Colibri/Query/Criteria.php
index 854ae00..2d161e4 100644
--- a/src/Colibri/Query/Criteria.php
+++ b/src/Colibri/Query/Criteria.php
@@ -4,7 +4,6 @@
 
 use Colibri\Query\Builder\Common;
 use Colibri\Query\Builder\Select;
-use Colibri\Query\Builder\SelectInterface;
 use Colibri\Query\Builder\Syntax\GroupByTrait;
 use Colibri\Query\Builder\Syntax\HavingTrait;
 use Colibri\Query\Builder\Syntax\JoinTrait;