[GH-6798] added a new api endpoint to get the bulk reactions for posts (#10049)

* 6798 added a new api to get the bulk reactions for posts

* 6798 added the permsission check before getting the reactions

* GH-6798 added a new app function for the new endpoint

* 6798 added a store method to get reactions for multiple posts

* 6798 connected the app function with the new store function

* 6798 fixed the review comments

app/reaction.go

@@ -60,6 +60,35 @@ func (a *App) GetReactionsForPost(postId string) ([]*model.Reaction, *model.AppE
 	return result.Data.([]*model.Reaction), nil
 }
 
+func (a *App) GetBulkReactionsForPosts(postIds []string) (map[string][]*model.Reaction, *model.AppError) {
+	reactions := make(map[string][]*model.Reaction)
+
+	result := <-a.Srv.Store.Reaction().BulkGetForPosts(postIds)
+	if result.Err != nil {
+		return nil, result.Err
+	}
+
+	allReactions := result.Data.([]*model.Reaction)
+	for _, reaction := range allReactions {
+		reactionsForPost := reactions[reaction.PostId]
+		reactionsForPost = append(reactionsForPost, reaction)
+
+		reactions[reaction.PostId] = reactionsForPost
+	}
+
+	reactions = populateEmptyReactions(postIds, reactions)
+	return reactions, nil
+}
+
+func populateEmptyReactions(postIds []string, reactions map[string][]*model.Reaction) map[string][]*model.Reaction {
+	for _, postId := range postIds {
+		if _, present := reactions[postId]; !present {
+			reactions[postId] = []*model.Reaction{}
+		}
+	}
+	return reactions
+}
+
 func (a *App) DeleteReactionForPost(reaction *model.Reaction) *model.AppError {
 	post, err := a.GetSinglePost(reaction.PostId)
 	if err != nil {