add real offering/seeking counts to item page

src/app/ItemPage.js

@@ -552,22 +552,54 @@ function ItemPageWantButton({ itemId, isChecked }) {
 }
 
 function ItemPageTradeLinks({ itemId }) {
+  const { data, loading, error } = useQuery(
+    gql`
+      query ItemPageTradeLinks($itemId: ID!) {
+        item(id: $itemId) {
+          id
+          numUsersOfferingThis
+          numUsersSeekingThis
+        }
+      }
+    `,
+    { variables: { itemId } }
+  );
+
+  if (error) {
+    return <Box color="red.400">{error.message}</Box>;
+  }
+
   return (
-    <Tooltip label="These are placeholders—real trade lists coming soon!">
     <HStack spacing="2">
       <Box as="header" fontSize="sm" fontWeight="bold">
         Trading:
       </Box>
-        <ItemPageTradeLink count={8} label="offering" colorScheme="green" />
+      <SubtleSkeleton isLoaded={!loading}>
-        <ItemPageTradeLink count={12} label="seeking" colorScheme="blue" />
+        <ItemPageTradeLink
+          itemId={itemId}
+          count={data?.item?.numUsersOfferingThis || 0}
+          label="offering"
+          colorScheme="green"
+        />
+      </SubtleSkeleton>
+      <SubtleSkeleton isLoaded={!loading}>
+        <ItemPageTradeLink
+          itemId={itemId}
+          count={data?.item?.numUsersSeekingThis || 0}
+          label="seeking"
+          colorScheme="blue"
+        />
+      </SubtleSkeleton>
     </HStack>
-    </Tooltip>
   );
 }
 
-function ItemPageTradeLink({ count, label, colorScheme }) {
+function ItemPageTradeLink({ itemId, count, label, colorScheme }) {
   return (
     <Button
+      as="a"
+      // TODO: Link to a new Impress 2020 page instead!
+      href={`https://impress.openneo.net/items/${itemId}`}
       size="xs"
       variant="outline"
       colorScheme={colorScheme}

src/server/loaders.js

@@ -388,6 +388,56 @@ const buildItemAllOccupiedZonesLoader = (db) =>
     );
   });
 
+const buildItemTradeCountsLoader = (db) =>
+  new DataLoader(
+    async (itemIdOwnedPairs) => {
+      const qs = itemIdOwnedPairs
+        .map((_) => "(closet_hangers.item_id = ? AND closet_hangers.owned = ?)")
+        .join(" OR ");
+      const values = itemIdOwnedPairs
+        .map(({ itemId, isOwned }) => [itemId, isOwned])
+        .flat();
+      const [rows, _] = await db.execute(
+        `
+        SELECT
+          items.id AS item_id, closet_hangers.owned AS is_owned,
+            count(DISTINCT closet_hangers.user_id) AS users_count
+          FROM items
+          INNER JOIN closet_hangers ON closet_hangers.item_id = items.id
+          INNER JOIN users ON users.id = closet_hangers.user_id
+          LEFT JOIN closet_lists ON closet_lists.id = closet_hangers.list_id
+          WHERE (
+            (${qs})
+            AND (
+              (closet_hangers.list_id IS NOT NULL AND closet_lists.visibility >= 2)
+              OR (
+                closet_hangers.list_id IS NULL AND closet_hangers.owned = 1
+                AND users.owned_closet_hangers_visibility >= 2
+              )
+              OR (
+                closet_hangers.list_id IS NULL AND closet_hangers.owned = 0
+                AND users.wanted_closet_hangers_visibility >= 2
+              )
+            )
+          )
+          GROUP BY items.id, closet_hangers.owned;
+      `,
+        values
+      );
+
+      const entities = rows.map(normalizeRow);
+
+      return itemIdOwnedPairs.map(({ itemId, isOwned }) => {
+        // NOTE: There may be no matching row, if there are 0 such trades.
+        const entity = entities.find(
+          (e) => e.itemId === itemId && Boolean(e.isOwned) === isOwned
+        );
+        return entity ? entity.usersCount : 0;
+      });
+    },
+    { cacheKeyFn: ({ itemId, isOwned }) => `${itemId}-${isOwned}` }
+  );
+
 const buildPetTypeLoader = (db, loaders) =>
   new DataLoader(async (petTypeIds) => {
     const qs = petTypeIds.map((_) => "?").join(",");
@@ -811,6 +861,7 @@ function buildLoaders(db) {
     db
   );
   loaders.itemAllOccupiedZonesLoader = buildItemAllOccupiedZonesLoader(db);
+  loaders.itemTradeCountsLoader = buildItemTradeCountsLoader(db);
   loaders.petTypeLoader = buildPetTypeLoader(db, loaders);
   loaders.petTypeBySpeciesAndColorLoader = buildPetTypeBySpeciesAndColorLoader(
     db,

src/server/types/Item.js

@@ -22,6 +22,10 @@ const typeDefs = gql`
     currentUserOwnsThis: Boolean!
     currentUserWantsThis: Boolean!
 
+    # How many users are offering/seeking this in their public trade lists.
+    numUsersOfferingThis: Int!
+    numUsersSeekingThis: Int!
+
     # How this item appears on the given species/color combo. If it does not
     # fit the pet, we'll return an empty ItemAppearance with no layers.
     appearanceOn(speciesId: ID!, colorId: ID!): ItemAppearance!
@@ -185,6 +189,21 @@ const resolvers = {
       return closetHangers.some((h) => h.itemId === id && !h.owned);
     },
 
+    numUsersOfferingThis: async ({ id }, _, { itemTradeCountsLoader }) => {
+      const count = await itemTradeCountsLoader.load({
+        itemId: id,
+        isOwned: true,
+      });
+      return count;
+    },
+    numUsersSeekingThis: async ({ id }, _, { itemTradeCountsLoader }) => {
+      const count = await itemTradeCountsLoader.load({
+        itemId: id,
+        isOwned: false,
+      });
+      return count;
+    },
+
     appearanceOn: async (
       { id },
       { speciesId, colorId },