Two behaviors of value template.

[ATaiIsHere]

I have a MESSAGE model to store messages for users.
I hope GSI1PK, GSI1SK are created when isRead is false, so I write my schema like this.

// schema
{
  version: '0.0.1',
  indexes: {
    'primary': { hash: 'pk', sort: 'sk' },
    'gsi-1': { hash: 'GSI1PK', sort: 'GSI1SK', project: 'all' },
  },
  models: {
    MESSAGE: {
      pk: { type: String, value: `USER#${userId}` },
      sk: { type: String, value: `MESSAGE#${id}` },
      GSI1PK: { type: String, value: `USER#${userId}` },
      GSI1SK: { type: String, value: true },
      GSI1SK: { type: String, value: true },
      userId: { type: String, required: true },
      id: { type: String, required: true, generate: 'ulid' },
      isRead: { type: Boolean, default: false },
    }
  },
};

// value
export default (
  model: AnyModel,
  fieldName: string,
  properties: OneProperties,
  params?: OneParams
): any => {
  if (model.name === 'MESSAGE') {
    if (fieldName === 'GSI1PK') {
      return properties?.isRead === false
        ? `USER#${properties.userId}`
        : properties?.isRead === true
        ? null
        : undefined;
    } else if (fieldName === 'GSI1SK') {
      return properties?.isRead === false
        ? `UNREAD`
        : properties?.isRead === true
        ? null
        : undefined;
    }
  }
  return undefined;
};

When use gsi-1 to find message ( like model.find({ GSI1PK: 'USER#1', GSI1SK: 'UNREAD' }, { index: 'gsi-1' }) ) ,
I got a MissingError as Empty hash key. Check hash key and any value template variable references,

https://github.com/sensedeep/dynamodb-onetable/blob/main/src/Model.js#L1400
because it will call default function to build GSI1PK, GSI1SK anyway (line 1409).
but set value to a string template (line 1422), it will check properties[name] === undefined first.

why is there such a difference?

this makes me need to think more about I can use GSI1PK, GSI1SK to query directly or not.

[mobsense]

You need to provide the userId that is required by the value template for GSI1PK.

Also, if you put isRead in the value template, then the GSI entry would only get created when the value is true and you would not need to provide the GSI values to the model.find. Best practice is to use value templates to create the PK/SK, GSI PK/SK values for you.