Just use _source instead of _source + fields

src/compiler.rs

 }
 
 pub fn compile(expr: Select) -> Result<Query, CompilerError> {
-    let mut source = false;
     let field_names: Vec<Value> = expr
         .fields
         .iter()
         .map(|f| match f {
-            Field::Star => {
-                source = true;
-                Value::String("*".to_string())
-            }
+            Field::Star => Value::String("*".to_string()),
             Field::Named(name) => Value::String(name.clone()),
         })
         .collect();
 
     let body = json!({
         "query": { "match_all":{} },
-        "_source": source,
-        "fields": Value::Array(field_names)
+        "_source": Value::Array(field_names)
     });
 
     Ok(Query::new(expr.source, body))
                     "query": {
                         "match_all": {}
                     },
-                    "fields": ["*"],
-                    "_source": true
+                    "_source": ["*"]
                 })
             )
         )
                     "query": {
                         "match_all": {}
                     },
-                    "fields": ["foo"],
-                    "_source": false
+                    "_source": ["foo"]
                 })
             )
         )