/// Get the total notification count by repos.

    pub fn count_by_repos(

        &self,

    ) -> Result<impl Iterator<Item = Result<(RepoId, usize), Error>> + '_, Error> {

        let stmt = self.db.prepare(

            "SELECT repo, COUNT(*) as count

            FROM `repository-notifications`

            GROUP BY repo",

        )?;

        Ok(stmt.into_iter().map(|row| {

            let row = row?;

            let count = row.try_read::<i64, _>("count")? as usize;

            let repo = row.try_read::<RepoId, _>("repo")?;

            Ok((repo, count))

        }))

    }

    /// Get the notification count for the given repo.

    fn test_count_by_repos() {

        let mut db = Store::open(":memory:").unwrap();

        let repo1 = arbitrary::gen::<RepoId>(1);

        let repo2 = arbitrary::gen::<RepoId>(1);

        let oid = arbitrary::oid();

        let time = LocalTime::from_millis(32188142);

        let update1 = RefUpdate::Created {

            name: refname!("refs/heads/feature/1"),

            oid,

        };

        let update2 = RefUpdate::Created {

            name: refname!("refs/heads/feature/2"),

            oid,

        };

        let update3 = RefUpdate::Created {

            name: refname!("refs/heads/feature/3"),

            oid,

        };

        assert!(db.insert(&repo1, &update1, time).unwrap());

        assert!(db.insert(&repo1, &update2, time).unwrap());

        assert!(db.insert(&repo2, &update3, time).unwrap());

        let mut count = db.count_by_repos().unwrap();

        assert_eq!(count.next().unwrap().unwrap(), (repo1, 2));

        assert_eq!(count.next().unwrap().unwrap(), (repo2, 1));

    }

    #[test]