How to avoid implicit 'null' substitution in GraphQL (AWS App Sync)?

Question

Consider below query:

const fetchItemsQuery = `query(
  $paginationPageSize: Int! 
  $paginationPageNumber: Int!
  $searchByNameRequest: String
  $categoryID: String
) {
  getItems(
    paginationPageSize: $paginationPageSize
    paginationPageNumber: $paginationPageNumber
    searchByNameRequest: $searchByNameRequest
    categoryID: $categoryID
  ) {
    // ...
  }
};

If to submit it from client side with variables:

{
  paginationPageSize: 20,
  paginationPageNumber: 1
}

in AWS App Sync logs, it reads:

{
  paginationPageSize: 20,
  paginationPageNumber: 1,
  searchByNameRequest: null,
  categoryID: null
}

I checked the request payload from browser - no nulls, just

{
  paginationPageSize: 20,
  paginationPageNumber: 1
}

So the cause is not in frontend side.

I found out that the null substitution occurring in request resolving template:

{
  "version" : "2017-02-28",
  "operation": "Invoke",
  "payload": {
    "field": "listProduct",
    "arguments": $util.toJson($context.args) // <- here
  }
}

$util.toJson uses Apache Velocity.

Can I avoid $util.toJson($context.args) will substitute the nulls in my request variables?

Important: the solution should not touch the explicitly submitted nulls .

did you end up resolving this? It seems pretty bad of appsync to not think that null has meaning in GraphQL compared to undefined or non-present. The defaults option below seems like a hacky workaround — Daniel Blignaut
– Daniel Blignaut, Commented Aug 26, 2021 at 15:22

Claude Brisson · Accepted Answer · 2020-06-03 03:14:52Z

1

You could try to filter out null arguments from the $context.args map.

#set( $args = {} )
#foreach( $entry in $context.args.entrySet())
  #if( $entry.value != $null )
    #set( $junk = $args.put($entry.key, $entry.value) )
  #end
#end

{
  "version" : "2017-02-28",
  "operation": "Invoke",
  "payload": {
    "field": "listProduct",
    "arguments": $util.toJson($args)
  }
}

answered Jun 3, 2020 at 3:14

Claude Brisson

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2 Comments

Takeshi Tokugawa YD Over a year ago

Thank you for the answer! But what if null has been submitted explicitly?

Claude Brisson Over a year ago

@GurebuBokofu You could assign default values like ` = "DEFAULT"` to GraphQL arguments, and check for this value to spot unspecified arguments.

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