Add indexed_fields method on Model

For postgres indexes we want to know the indexed field names upfront
without an actual instance of Model

Signed-off-by: Lee Smet <lee.smet@hotmail.com>

heromodels-derive/src/lib.rs

@@ -123,6 +123,11 @@ pub fn model(_attr: TokenStream, item: TokenStream) -> TokenStream {
         }
     };
 
+    let indexed_field_names = indexed_fields
+        .iter()
+        .map(|f| f.0.to_string())
+        .collect::<Vec<_>>();
+
     let model_impl = quote! {
         impl heromodels_core::Model for #struct_name {
             fn db_prefix() -> &'static str {
@@ -137,6 +142,12 @@ pub fn model(_attr: TokenStream, item: TokenStream) -> TokenStream {
                 &mut self.base_data
             }
 
+            fn indexed_fields() -> Vec<&'static str> {
+                vec![
+                    #(#indexed_field_names),*
+                ]
+            }
+
             #db_keys_impl
         }
     };