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The form has a Customer document and we have specified URI template .../customers/{customer} and Read method GET. In this form, there is a control which has Type ID 'customer' as shown in the figure below.
If you enter a value in this control, will automatically resolve the URI template using the new value and attempt to GET a new document. If it succeeds, the form will be initialized with the new document and all relevant control values will automatically update. In the example above, try changing the value to 03 or 04. Notice how the customer information fields change to reflect the new document that is being edited by the form.
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