Make get_relation_items const and comment that update_reference_items…

… behaves as const.

cxx/gendb.cc

@@ -170,7 +170,7 @@ T_items GenDB::sample_class_ancestors(std::mt19937* prng,
 // class_item equal to a primary key value (which goes in the last element of
 // items and determines the rest of the values).
 void GenDB::get_relation_items(const std::string& rel_name, const int ind,
-                               const int class_item, T_items& items) {
+                               const int class_item, T_items& items) const {
   const std::vector<std::string>& domains = std::visit(
       [&](auto tr) { return tr.domains; }, hirm->schema.at(rel_name));
   items[ind] = class_item;

cxx/gendb.hh

@@ -66,7 +66,7 @@ class GenDB {
   // Populates "items" with entities by walking the DAG of reference indices,
   // starting with "ind".
   void get_relation_items(const std::string& rel_name, const int ind,
-                          const int class_item, T_items& items);
+                          const int class_item, T_items& items) const;
 
   // Unincorporates all data where class_name refers to ref_field amd has
   // primary key value class_item.
@@ -88,7 +88,8 @@ class GenDB {
   // Returns a copy of stored_values, with the items updated to associate
   // class_name.ref_field at index class_item with new_ref_val. This update is
   // recursive, such that the reference fields of the class corresponding to
-  // ref_field are updated as well.
+  // ref_field are updated as well. This method behaves as const (although it
+  // modifies/restores the state of reference_values).
   std::map<std::string,
            std::unordered_map<T_items, ObservationVariant, H_items>>
   update_reference_items(