Intro TypedKey and TypedProperty

Those are `Key`s and `Property`s that statically know the value type
they refer too. For example `CodeAttribute`'s.

Sources/ZeeQL/Access/CodeAttribute.swift

@@ -14,7 +14,7 @@
  * Their primary feature over `ModelAttribute`s is that such are generic over
  * the value type.
  */
-open class CodeAttribute<T: AttributeValue> : ModelAttribute {
+open class CodeAttribute<T: AttributeValue> : ModelAttribute, TypedProperty {
 
   // Note: we remove the keyword
   @inlinable

Sources/ZeeQL/Access/CodeRelationship.swift

@@ -192,8 +192,11 @@ open class CodeRelationshipBase<Target: SwiftObject>
 }
 
 open class CodeRelationship<Target: DatabaseObject>
-             : CodeRelationshipBase<Target>, CodeRelationshipType
+             : CodeRelationshipBase<Target>, CodeRelationshipType,
+               TypedProperty
 {
+  public typealias T = Target
+
   public var codeEntity : Entity? = nil
   override open var entity : Entity {
     set {

Sources/ZeeQL/Access/Property.swift

@@ -3,11 +3,11 @@
 //  ZeeQL
 //
 //  Created by Helge Heß on 18.02.17.
-//  Copyright © 2017-2021 ZeeZide GmbH. All rights reserved.
+//  Copyright © 2017-2024 ZeeZide GmbH. All rights reserved.
 //
 
 /**
- * A property of an `Entity`: Either an `Attribute` or a `Relationship`.
+ * A property of an ``Entity``: Either an ``Attribute`` or a ``Relationship``.
  */
 public protocol Property : AnyObject, EquatableType {
   // `class` because we use identity in some places
@@ -16,3 +16,11 @@ public protocol Property : AnyObject, EquatableType {
   var relationshipPath : String? { get }
 
 }
+
+/**
+ * A property of an ``Entity``: Either an ``Attribute`` or a ``Relationship``,
+ * that knows the static type of the property.
+ */
+public protocol TypedProperty<T>: Property {
+  associatedtype T
+}

Sources/ZeeQL/Control/Key.swift

@@ -33,6 +33,10 @@ public protocol Key : Expression, ExpressionEvaluation, EquatableType,
   func append(_ key: Key) -> Key
 }
 
+public protocol TypedKey<T>: Key {
+  associatedtype T
+}
+
 public extension Key {
 
   // MARK: - value