Add download counter on each book

* Closes: #40

lib/database/database.go

@@ -29,6 +29,7 @@ type DB interface {
 	GetNews(num int, days int) (news []New, err error)
 	IncViews(ID string) error
 	IncDownloads(ID string) error
+	GetDownloadCounter(ID string) (int, error)
 	GetFrontPage() FrontPage
 	AddSubmission(submission Submission) (id int, err error)
 	UpdateSubmission(id int, status string, book *Book) error
@@ -222,6 +223,7 @@ $$ LANGUAGE sql IMMUTABLE;
 -- Visits indexes
 CREATE INDEX IF NOT EXISTS visits_downloads_idx on visits(downloads DESC NULLS LAST);
 CREATE INDEX IF NOT EXISTS visits_views_idx on visits(views DESC NULLS LAST);
+CREATE INDEX IF NOT EXISTS visits_book_id_idx on visits(book_id);
 
 -- Submissions indexes
 CREATE INDEX IF NOT EXISTS submissions_id_idx on submissions(submission_id);

lib/database/ro.go

@@ -93,6 +93,10 @@ func (db *roDB) IncDownloads(ID string) error {
 	return nil
 }
 
+func (db *roDB) GetDownloadCounter(ID string) (int, error) {
+	return db.db.GetDownloadCounter(ID)
+}
+
 func (db *roDB) GetFrontPage() FrontPage {
 	return db.db.GetFrontPage()
 }

lib/database/stats.go

@@ -40,6 +40,15 @@ func (db *pgDB) IncDownloads(ID string) error {
 	return err
 }
 
+func (db *pgDB) GetDownloadCounter(ID string) (int, error) {
+	var num int
+	err := db.sql.Model(&Visit{}).
+		Column("downloads").
+		Where("book_id = ?", ID).
+		Select(&num)
+	return num, err
+}
+
 func (db *pgDB) GetFrontPage() FrontPage {
 	return db.frontPage
 }