Azure Function App cosmosdb input binding - read from nested json

I have the following json sent to an Azure Function App:

{
"a" : {
  "nested" : {
    "object" : "hello_world"
    }
  }
}

Within the function.json I would like to fetch a document based on the object value (hello_world in this example) :

{
  "scriptFile": "__init__.py",
  "bindings": [
    {
      "authLevel": "function",
      "type": "httpTrigger",
      "direction": "in",
      "name": "req",
      "methods": [
        "post"
      ]
    },
    {
      "type": "cosmosDB",
      "direction": "in",
      "name": "doc_in",
      "databaseName": "my_db",
      "collectionName": "my_collection",
      "connectionStringSetting": "AZURE_COSMOS_DB_CONNECTION_STRING",
      "sqlQuery": "SELECT * FROM my_db as db WHERE db.my_field = {a.nested.object}"
    },
    {
      "type": "http",
      "direction": "out",
      "name": "$return"
    }
  ]
}

init.py

def main(req: func.HttpRequest, doc_in: func.In[func.Document]) -> func.HttpResponse:
    if doc_in:
        return func.HttpResponse(
            json.dumps({"msg":"A document was found!"}),
            status_code=200,
            mimetype="application/json" 
            )  

The SELECT * FROM my_db as db WHERE db.my_field = {a.nested.object} query does not work and return a HTTP 404 code and there's no much information in the documentation.

Does someone know how am I supposed to set the input json file into the cosmos query ?