Can't conditionally update custom mapped list of strings with empty value

Describe the bug

If I have a property with a custom mapper like

@Model()
class MyModel {
  @Property({ mapper: stringArrayOrEmptyMapper })
  myList: string[]
}

export const stringArrayOrEmptyMapper: MapperForType<
  string[],
  ListAttribute<StringAttribute>
> = {
  fromDb: attributeValue =>
    attributeValue?.L?.map((item: { S: string }) => item.S) ?? [],
  toDb: propertyValue =>
    propertyValue?.length
      ? { L: propertyValue.map(item => ({ S: item })) }
      : { L: [] },
};

and I try to do a conditional transact update like:

new TransactUpdate(MyModel, PK, SK)
  .onlyIfAttribute('myList')
  .eq([])
  .updateAttribute('myList)
  .set(['liststuff'])

I get Error: expected 1 value(s) for operator =, this is not the right amount of method parameters for this operator

This works fine if the list is populated but fails when the list is empty

Expected behavior
I expect the operation to succeed if the list is empty and fail if the list is not empty. I do not expect an error

Seems like this is due to line 310 in condition-expression-builder.ts. The validator throws if the conditional values length is falsey. Why is this needed?

function validateForOperator(operator: ConditionOperator, values?: any[]) {
  validateArity(operator, values)

  /*
   * validate values if operator supports values
   */
  if (!isFunctionOperator(operator) || (isFunctionOperator(operator) && !isNoParamFunctionOperator(operator))) {
    if (values && Array.isArray(values) && values.length) {
      validateValues(operator, values)
    } else {
      throw new Error(
        dynamicTemplate(ERR_ARITY_DEFAULT, { parameterArity: operatorParameterArity(operator), operator }),
      )
    }
  }
}

Also empty arrays are striped by the deepFilter in buildFilterExpression in condition-expression-builder.ts. Is this necessary?

export function buildFilterExpression(
  attributePath: string,
  operator: ConditionOperator,
  values: any[],
  existingValueNames: string[] | undefined,
  metadata: Metadata<any> | undefined,
): Expression {
  // metadata get rid of undefined values
  values = deepFilter(values, (value) => value !== undefined)

This is legal to DynamoDB

TransactItems: [
   {
      Update: {
        TableName: ...,
        Key: ...,
        ConditionExpression: 'myList = :emptyList',
        UpdateExpression: 'SET myList = :myList',
        ExpressionAttributeValues: {
          ':emptyList': { L: [] },
          ':myList': { L: [{ S: 'value' }] },
        },
      },
    }
]

any progress on this issue? I'm experiencing the same problem which leads to an issue with GraphQL resolvers with nonNull lists

I ended up just using the using the dynamodb client directly for this specific use case.

This problem extends to all actions containing empty arrays, not just condition expressions. I can't see any reason deepFilter doesn't allow empty arrays. They are 100% valid in dynamoDB. Not sure this repo is actively maintained anymore though. Forked repo and fixed here: Fantom-App#1

fix(update): fix setting empty string arrays when running updates

Any changes the suggested fix will be added into this repo? Would be great 😄

@LucasDachman

@LucasDachman @stvngrsh @TanjaBayer A fix for this is released on the next stream -> ^8.0.0-next.7 #431 (comment)

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Can't conditionally update custom mapped list of strings with empty value #335

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Can't conditionally update custom mapped list of strings with empty value #335

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LucasDachman commented on Oct 19, 2020

danielblignaut commented on Jan 21, 2021

LucasDachman commented on Jan 21, 2021

stvngrsh commented on Oct 25, 2021

TanjaBayer commented on Feb 2, 2024

michaelwittwer commented on May 13, 2025

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