Accessing MatchedParameters from an if statement in set-body - azure-api-management

Question

In an APIM policy for a REST API, I'm trying to create a SOAP request for the backend service.
The URI of the REST request is /GetCustomer/{accountNumber}.
In the Liquid if statement below, I'm trying to check if the MatchedParameters collection contains the account number. This doesn't work, I get the value from the else branch.
I tried a couple of variations:
{% if {{context.Request.MatchedParameters.ContainsKey("accountNumber")}} %}
{% if #(context.Request.MatchedParameters.ContainsKey("accountNumber")) %}
{% if context.Request.MatchedParameters.ContainsKey("accountNumber") %}
<set-body template="liquid">
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns="http://my-services.com/CustomerService/1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soap:Body>
<GetCustomer>
<request>
{% if {{context.Request.MatchedParameters.ContainsKey("accountNumber")}} %}
<AccountNumber>{{context.Request.MatchedParameters["accountNumber"]}}</AccountNumber>
{% else %}
<AccountNumber xsi:nil="true" />
{% endif %}
</request>
</GetCustomer>
</soap:Body>
</soap:Envelope>
</set-body>
Leaving the if statement out works correctly, then the request is constructed as I expect:
<set-body template="liquid">
<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/" xmlns="http://my-services.com/CustomerService/1.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
<soap:Body>
<GetCustomer>
<request>
<AccountNumber>{{context.Request.MatchedParameters["accountNumber"]}}</AccountNumber>
</request>
</GetCustomer>
</soap:Body>
</soap:Envelope>
</set-body>
So the question is: is it possible to access the context from within a Liquid if statement? And if so, what is the correct syntax?

Answer 1

I test it in my side, you just need to write the liquid like:
{% if context.Request.MatchedParameters["accountNumber"] != null %}
{% else %}
{% endif %}
And by the way: According to my test, if you specify a {accountNumber} in the request url of APIM. We have to requsest it with ...../GetCustomer/{accountNumber}, if we request it with ...../GetCustomer, it will show 404 error. So you do not need to worry about "if context.Request.MatchedParameters["accountNumber"] is exist"