'An undeclared property' when trying to create record via Web API

Question

I am getting an error which I just cannot seem to debug. I am trying to create a custom activity entity via custom HTML/JavaScript web resource.

The user clicks a b

Answer 1

When you want to set the value of a lookup field during the creation or update of a (new) record via the web API, you have to use either the Schema Name or the Logical Name of the lookup followed by the bind annotation.

• For default fields like primarycontactid the logical name has to be used (first column in the screenshot).
• For custom fields like rob_FaqId the schema name has to be used (second column in the screenshot).

var params = {
    'rob_FaqId@odata.bind': '/rob_faqs(guid-here)',
    'rob_source': 180840000,
    'subject': 'Signpost',
    'actualstart': new Date(),
    'actualend': new Date()
};

Screenshot of a solution > entities > your entity > fields:

So the general structure to create a new record with an already set lookup field via the web API is this:

{
  "logicalorschemaName@odata.bind": "/relatedentitys(guid)" //don't forget the plural 's'
}

Or another example from the official documentation. How to create a new account record and directly assign an already existing contact as the primary contact.

var newAccountRecordObj = {
  "name": "Sample Account",
  "primarycontactid@odata.bind": "/contacts(00000000-0000-0000-0000-000000000001)"
}