Update polymorphic-objects.md

articles/modeling/polymorphic-objects.md

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+# Polymorphic Objects 
 
+## What 
+
+- Resources in your API are polymorphic. They can be returned as XML or JSON and can have a flexible amount of fields. You can also have requests and responses in your API design that can be depicted by a number of alternative schemas. 
+- **Polymorphism** is the capacity to present the same interface for differing underlying forms. 
+- The **discriminator** keyword is used to designate the name of the property that decides which schema definition validates the structure of the model. 
+
+## Why 
+- Polymorphism permits combining and extending model definitions. 
+
+## Best Practices
+
+<!-- theme: warning -->
+>The discriminator property **must** be a mandatory or required field. When it is used, the value **must** be the name of the schema or any schema that inherits it. 
+
+### Example 
+
+```
+{
+  definitions:
+  Vehicle:
+    type: object, 
+    discriminator: brand
+    properties:
+      model:
+        type: string
+      color:
+        type: string
+      required:
+    model
+    
+  Sedan: # Sedan is used as the discriminator value 
+  
+    allOf:
+      $ref: '#/definitions/Vehicle'
+      type: object
+        properties:
+          dateManufactured: 
+            type: date
+          required: 
+            dateManufactured
+}
+```