store: Return latest value from WritableStore::get_many and get_derived

In cases where the queue had multiple entries, and an entity was modified
by more than one entry, we would return the oldest value in the queue
instead of the newest one from WritableStore::get_many and get_derived.

This change makes sure that we always return the correct (newest) value in
these situations

Author: lutter

store/postgres/src/writable.rs

@@ -1065,8 +1065,14 @@ impl Queue {
             &self.queue,
             BTreeMap::new(),
             |mut map: BTreeMap<EntityKey, Option<Entity>>, batch, at| {
-                // See if we have changes for any of the keys.
+                // See if we have changes for any of the keys. Since we are
+                // going from newest to oldest block, do not clobber already
+                // existing entries in map as that would make us use an
+                // older value.
                 for key in &keys {
+                    if map.contains_key(key) {
+                        continue;
+                    }
                     match batch.last_op(key, at) {
                         Some(EntityOp::Write { key: _, entity }) => {
                             map.insert(key.clone(), Some(entity.clone()));
@@ -1133,7 +1139,14 @@ impl Queue {
             &self.queue,
             BTreeMap::new(),
             |mut map: BTreeMap<EntityKey, Option<Entity>>, batch, at| {
-                map.extend(effective_ops(batch, derived_query, at));
+                // Since we are going newest to oldest, do not clobber
+                // already existing entries in map as that would make us
+                // produce stale values
+                for (k, v) in effective_ops(batch, derived_query, at) {
+                    if !map.contains_key(&k) {
+                        map.insert(k, v);
+                    }
+                }
                 map
             },
         );