nautechsystems/nautilus_trader · error

data_type must be a JSON object

Error message

data_type must be a JSON object

What it means

After the JSON parses, `from_persistence_json` requires the top-level value to be a JSON object (a map of fields for the DataType). If the string parses to a scalar, array, or null instead, this error is raised.

Source

Thrown at crates/model/src/data/mod.rs:808

                "identifier".to_string(),
                serde_json::Value::String(id.clone()),
            );
        }
        serde_json::to_string(&serde_json::Value::Object(map))
    }

    /// Deserializes from JSON produced by `to_persistence_json`.
    /// Accepts legacy JSON with `topic` (ignored); topic is rebuilt from `type_name` + metadata.
    ///
    /// # Errors
    ///
    /// Returns an error if the string is not valid JSON or missing required fields.
    pub fn from_persistence_json(s: &str) -> Result<Self, anyhow::Error> {
        let value: serde_json::Value =
            serde_json::from_str(s).map_err(|e| anyhow::anyhow!("Invalid data_type JSON: {e}"))?;
        let obj = value
            .as_object()
            .ok_or_else(|| anyhow::anyhow!("data_type must be a JSON object"))?;
        let type_name = obj
            .get("type_name")
            .and_then(|v| v.as_str())
            .ok_or_else(|| anyhow::anyhow!("data_type must have type_name"))?;
        let metadata = obj.get("metadata").and_then(|m| {
            if m.is_null() {
                None
            } else {
                let p: Params = serde_json::from_value(m.clone()).ok()?;
                if p.is_empty() { None } else { Some(p) }
            }
        });
        let identifier = obj
            .get("identifier")
            .and_then(|v| v.as_str())
            .map(String::from);
        Ok(Self::new(type_name, metadata, identifier))
    }

View on GitHub (pinned to 18893faf8b)

Solutions

1. Ensure the persisted form is a full object: `{"type_name": "...", "metadata": {...}}`, not just the type name.
2. Re-persist the DataType with the current library's serializer so the on-disk format matches.
3. Check that you're loading the whole record and not an inner field of it.
4. Validate the parsed value's shape before calling, e.g. `serde_json::from_str::<serde_json::Value>(s)?.is_object()`.

Example fix

// before
let dt = DataType::from_persistence_json(r#""QuoteTick""#)?;

// after
let dt = DataType::from_persistence_json(r#"{"type_name": "QuoteTick"}"#)?;

fn is_object_json(s: &str) -> bool {
    serde_json::from_str::<serde_json::Value>(s).map(|v| v.is_object()).unwrap_or(false)
}

fn as_datatype_object(v: &serde_json::Value) -> Option<&serde_json::Map<String, serde_json::Value>> {
    v.as_object()
}

When it happens

Trigger: Passing JSON like `"SomeType"`, `["a","b"]`, `123`, or `null` to `DataType::from_persistence_json` — anything that is valid JSON but not an object.

Common situations: Persisting only the type_name string and trying to reload it as a full DataType; a serialization change between versions that wrote arrays/scalars; slicing the wrong portion of a JSON document.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

• Invalid data_type JSON: {e}
• data_type must have type_name
• Failed to concatenate stream batches: {e}
• Failed to merge custom data type metadata: {e}
• height must be positive, was {self.height}

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/b18d60e855e3ae74. Report an issue: GitHub.