diff --git a/src/message.rs b/src/message.rs
index a22dbf8..90651b1 100644
--- a/src/message.rs
+++ b/src/message.rs
@@ -18,6 +18,7 @@ mod support_test {
 #[derive(Clone, Debug)]
 enum MTTError {
     DocumentAlreadyExists(String),
+    DocumentFieldNotFound(String),
     DocumentNotFound(String),
 }
 
@@ -905,12 +906,71 @@ impl CreateDoc {
     }
 }
 
+enum FieldType {
+    StaticString,
+    UUID,
+}
+
 #[derive(Clone, Debug)]
-struct DocDef;
+struct DocDef {
+    fields: Vec<String>,
+}
 
 impl DocDef {
     fn new() -> Self {
-        Self {}
+        Self { fields: Vec::new() }
+    }
+
+    fn add_field(&mut self, name: String) {
+        self.fields.push(name);
+    }
+
+    fn get_field(&self, name: &str) -> Result<String, MTTError> {
+        if self.fields.contains(&name.to_string()) {
+            Ok(name.to_string())
+        } else {
+            Err(MTTError::DocumentFieldNotFound(name.to_string()))
+        }
+    }
+}
+
+#[cfg(test)]
+mod docdefs {
+    use super::*;
+
+    #[test]
+    fn can_field_be_added() {
+        let mut docdef = DocDef::new();
+        let name = Uuid::new_v4().to_string();
+        docdef.add_field(name.clone());
+        let result = docdef.get_field(name.as_str()).unwrap();
+        assert_eq!(result, name);
+    }
+
+    #[test]
+    fn produces_error_for_bad_fields() {
+        let docdef = DocDef::new();
+        let name = Uuid::new_v4().to_string();
+        match docdef.get_field(name.as_str()) {
+            Ok(_) => unreachable!("should return non existant field error"),
+            Err(err) => match err {
+                MTTError::DocumentFieldNotFound(data) => assert_eq!(data, name),
+                _ => unreachable!("got {:?}: should have been document field not found", err),
+            },
+        }
+    }
+
+    #[test]
+    fn can_multiple_fields_be_added() {
+        let mut docdef = DocDef::new();
+        let names = ["one", "two", "three"];
+        for name in names.iter() {
+            docdef.add_field(name.to_string());
+        }
+        for name in names.iter() {
+            let result = docdef.get_field(name).unwrap();
+            assert_eq!(result, name.to_string());
+        }
     }
 }