diff --git a/core/attributedef/AttributeExternalKey.php b/core/attributedef/AttributeExternalKey.php
new file mode 100644
index 000000000..3be7a2989
--- /dev/null
+++ b/core/attributedef/AttributeExternalKey.php
@@ -0,0 +1,335 @@
+<?php
+
+/**
+ * Map a foreign key to an attribute
+ *  AttributeExternalKey and AttributeExternalField may be an external key
+ *  the difference is that AttributeExternalKey corresponds to a column into the defined table
+ *  where an AttributeExternalField corresponds to a column into another table (class)
+ *
+ * @package     iTopORM
+ */
+class AttributeExternalKey extends AttributeDBFieldVoid
+{
+    const SEARCH_WIDGET_TYPE = self::SEARCH_WIDGET_TYPE_EXTERNAL_KEY;
+
+    /**
+     * Useless constructor, but if not present PHP 7.4.0/7.4.1 is crashing :( (N°2329)
+     *
+     * @see https://www.php.net/manual/fr/language.oop5.decon.php states that child constructor can be ommited
+     * @see https://bugs.php.net/bug.php?id=79010 bug solved in PHP 7.4.9
+     *
+     * @param string $sCode
+     * @param array $aParams
+     *
+     * @throws \Exception
+     * @noinspection SenselessProxyMethodInspection
+     */
+    public function __construct($sCode, $aParams)
+    {
+        parent::__construct($sCode, $aParams);
+    }
+
+    /**
+     * Return the search widget type corresponding to this attribute
+     *
+     * @return string
+     */
+    public function GetSearchType()
+    {
+        try {
+            $oRemoteAtt = $this->GetFinalAttDef();
+            $sTargetClass = $oRemoteAtt->GetTargetClass();
+            if (MetaModel::IsHierarchicalClass($sTargetClass)) {
+                return self::SEARCH_WIDGET_TYPE_HIERARCHICAL_KEY;
+            }
+
+            return self::SEARCH_WIDGET_TYPE_EXTERNAL_KEY;
+        } catch (CoreException $e) {
+        }
+
+        return self::SEARCH_WIDGET_TYPE_RAW;
+    }
+
+    public static function ListExpectedParams()
+    {
+        return array_merge(parent::ListExpectedParams(), array("targetclass", "is_null_allowed", "on_target_delete"));
+    }
+
+    public function GetEditClass()
+    {
+        return "ExtKey";
+    }
+
+    protected function GetSQLCol($bFullSpec = false)
+    {
+        return "INT(11)" . ($bFullSpec ? " DEFAULT 0" : "");
+    }
+
+    public function RequiresIndex()
+    {
+        return true;
+    }
+
+    public function IsExternalKey($iType = EXTKEY_RELATIVE)
+    {
+        return true;
+    }
+
+    public function GetTargetClass($iType = EXTKEY_RELATIVE)
+    {
+        return $this->Get("targetclass");
+    }
+
+    public function GetKeyAttDef($iType = EXTKEY_RELATIVE)
+    {
+        return $this;
+    }
+
+    public function GetKeyAttCode()
+    {
+        return $this->GetCode();
+    }
+
+    public function GetDisplayStyle()
+    {
+        return $this->GetOptional('display_style', 'select');
+    }
+
+
+    public function GetDefaultValue(DBObject $oHostObject = null)
+    {
+        return 0;
+    }
+
+    public function IsNullAllowed()
+    {
+        if (MetaModel::GetConfig()->Get('disable_mandatory_ext_keys')) {
+            return true;
+        }
+
+        return $this->Get("is_null_allowed");
+    }
+
+
+    public function GetBasicFilterOperators()
+    {
+        return parent::GetBasicFilterOperators();
+    }
+
+    public function GetBasicFilterLooseOperator()
+    {
+        return parent::GetBasicFilterLooseOperator();
+    }
+
+    public function GetBasicFilterSQLExpr($sOpCode, $value)
+    {
+        return parent::GetBasicFilterSQLExpr($sOpCode, $value);
+    }
+
+    // overloaded here so that an ext key always have the answer to
+    // "what are your possible values?"
+    public function GetValuesDef()
+    {
+        $oValSetDef = $this->Get("allowed_values");
+        if (!$oValSetDef) {
+            // Let's propose every existing value
+            $oValSetDef = new ValueSetObjects('SELECT ' . $this->GetTargetClass());
+        }
+
+        return $oValSetDef;
+    }
+
+    public function GetAllowedValues($aArgs = array(), $sContains = '')
+    {
+        //throw new Exception("GetAllowedValues on ext key has been deprecated");
+        try {
+            return parent::GetAllowedValues($aArgs, $sContains);
+        } catch (Exception $e) {
+            // Some required arguments could not be found, enlarge to any existing value
+            $oValSetDef = new ValueSetObjects('SELECT ' . $this->GetTargetClass());
+
+            return $oValSetDef->GetValues($aArgs, $sContains);
+        }
+    }
+
+    public function GetAllowedValuesForSelect($aArgs = array(), $sContains = '')
+    {
+        //$this->GetValuesDef();
+        $oValSetDef = new ValueSetObjects('SELECT ' . $this->GetTargetClass());
+        return $oValSetDef->GetValuesForAutocomplete($aArgs, $sContains);
+    }
+
+
+    public function GetAllowedValuesAsObjectSet($aArgs = array(), $sContains = '', $iAdditionalValue = null)
+    {
+        $oValSetDef = $this->GetValuesDef();
+        $oSet = $oValSetDef->ToObjectSet($aArgs, $sContains, $iAdditionalValue);
+
+        return $oSet;
+    }
+
+    public function GetAllowedValuesAsFilter($aArgs = array(), $sContains = '', $iAdditionalValue = null)
+    {
+        return DBObjectSearch::FromOQL($this->GetValuesDef()->GetFilterExpression());
+    }
+
+    public function GetDeletionPropagationOption()
+    {
+        return $this->Get("on_target_delete");
+    }
+
+    public function GetNullValue()
+    {
+        return 0;
+    }
+
+    public function IsNull($proposedValue)
+    {
+        return ($proposedValue == 0);
+    }
+
+    /**
+     * @inheritDoc
+     */
+    public function HasAValue($proposedValue): bool
+    {
+        return ((int)$proposedValue) !== 0;
+    }
+
+    public function MakeRealValue($proposedValue, $oHostObj)
+    {
+        if (is_null($proposedValue)) {
+            return 0;
+        }
+        if ($proposedValue === '') {
+            return 0;
+        }
+        if (MetaModel::IsValidObject($proposedValue)) {
+            return $proposedValue->GetKey();
+        }
+
+        return (int)$proposedValue;
+    }
+
+    /** @inheritdoc  @since 3.1 */
+    public function WriteExternalValues(DBObject $oHostObject): void
+    {
+        $sTargetKey = $oHostObject->Get($this->GetCode());
+        $oFilter = DBSearch::FromOQL('SELECT `' . TemporaryObjectDescriptor::class . '` WHERE item_class=:class AND item_id=:id');
+        $oSet = new DBObjectSet($oFilter, [], ['class' => $this->GetTargetClass(), 'id' => $sTargetKey]);
+        while ($oTemporaryObjectDescriptor = $oSet->Fetch()) {
+            $oTemporaryObjectDescriptor->Set('host_class', get_class($oHostObject));
+            $oTemporaryObjectDescriptor->Set('host_id', $oHostObject->GetKey());
+            $oTemporaryObjectDescriptor->Set('host_att_code', $this->GetCode());
+            $oTemporaryObjectDescriptor->DBUpdate();
+        }
+    }
+
+    public function GetMaximumComboLength()
+    {
+        return $this->GetOptional('max_combo_length', MetaModel::GetConfig()->Get('max_combo_length'));
+    }
+
+    public function GetMinAutoCompleteChars()
+    {
+        return $this->GetOptional('min_autocomplete_chars', MetaModel::GetConfig()->Get('min_autocomplete_chars'));
+    }
+
+    /**
+     * @return int
+     * @since 3.0.0
+     */
+    public function GetMaxAutoCompleteResults(): int
+    {
+        return MetaModel::GetConfig()->Get('max_autocomplete_results');
+    }
+
+    public function AllowTargetCreation()
+    {
+        return $this->GetOptional('allow_target_creation', MetaModel::GetConfig()->Get('allow_target_creation'));
+    }
+
+    /**
+     * Find the corresponding "link" attribute on the target class, if any
+     *
+     * @return null | AttributeDefinition
+     * @throws \CoreException
+     */
+    public function GetMirrorLinkAttribute()
+    {
+        $oRet = null;
+        $sRemoteClass = $this->GetTargetClass();
+        foreach (MetaModel::ListAttributeDefs($sRemoteClass) as $sRemoteAttCode => $oRemoteAttDef) {
+            if (!$oRemoteAttDef->IsLinkSet()) {
+                continue;
+            }
+            if (!is_subclass_of($this->GetHostClass(),
+                    $oRemoteAttDef->GetLinkedClass()) && $oRemoteAttDef->GetLinkedClass() != $this->GetHostClass()) {
+                continue;
+            }
+            if ($oRemoteAttDef->GetExtKeyToMe() != $this->GetCode()) {
+                continue;
+            }
+            $oRet = $oRemoteAttDef;
+            break;
+        }
+
+        return $oRet;
+    }
+
+    public static function GetFormFieldClass()
+    {
+        return '\\Combodo\\iTop\\Form\\Field\\SelectObjectField';
+    }
+
+    public function MakeFormField(DBObject $oObject, $oFormField = null)
+    {
+        /** @var \Combodo\iTop\Form\Field\Field $oFormField */
+        if ($oFormField === null) {
+            // Later : We should check $this->Get('display_style') and create a Radio / Select / ... regarding its value
+            $sFormFieldClass = static::GetFormFieldClass();
+            $oFormField = new $sFormFieldClass($this->GetCode());
+        }
+
+        // Setting params
+        $oFormField->SetMaximumComboLength($this->GetMaximumComboLength());
+        $oFormField->SetMinAutoCompleteChars($this->GetMinAutoCompleteChars());
+        $oFormField->SetMaxAutoCompleteResults($this->GetMaxAutoCompleteResults());
+        $oFormField->SetHierarchical(MetaModel::IsHierarchicalClass($this->GetTargetClass()));
+        // Setting choices regarding the field dependencies
+        $aFieldDependencies = $this->GetPrerequisiteAttributes();
+        if (!empty($aFieldDependencies)) {
+            $oTmpAttDef = $this;
+            $oTmpField = $oFormField;
+            $oFormField->SetOnFinalizeCallback(function () use ($oTmpField, $oTmpAttDef, $oObject) {
+                /** @var $oTmpField \Combodo\iTop\Form\Field\Field */
+                /** @var $oTmpAttDef \AttributeDefinition */
+                /** @var $oObject \DBObject */
+
+                // We set search object only if it has not already been set (overrided)
+                if ($oTmpField->GetSearch() === null) {
+                    $oSearch = DBSearch::FromOQL($oTmpAttDef->GetValuesDef()->GetFilterExpression());
+                    $oSearch->SetInternalParams(array('this' => $oObject));
+                    $oTmpField->SetSearch($oSearch);
+                }
+            });
+        } else {
+            $oSearch = DBSearch::FromOQL($this->GetValuesDef()->GetFilterExpression());
+            $oSearch->SetInternalParams(array('this' => $oObject));
+            $oFormField->SetSearch($oSearch);
+        }
+
+        parent::MakeFormField($oObject, $oFormField);
+
+        return $oFormField;
+    }
+
+    public function GetAsHTML($sValue, $oHostObject = null, $bLocalize = true)
+    {
+        if (!is_null($oHostObject)) {
+            return $oHostObject->GetAsHTML($this->GetCode(), $oHostObject);
+        }
+
+        return DBObject::MakeHyperLink($this->GetTargetClass(), $sValue);
+    }
+}
\ No newline at end of file