def create_db_in_shard(db_name, shard, client=None):
    """
    In a sharded cluster, create a database in a particular shard.
    """
    client = client or pymongo.MongoClient()
    # flush the router config to ensure it's not stale
    res = client.admin.command('flushRouterConfig')
    if not res.get('ok'):
        raise RuntimeError("unable to flush router config")
    if shard not in get_ids(client.config.shards):
        raise ValueError(f"Unknown shard {shard}")
    if db_name in get_ids(client.config.databases):
        raise ValueError("database already exists")
    # MongoDB doesn't have a 'create database' command, so insert an
    #  item into a collection and then drop the collection.
    client[db_name].foo.insert({'foo': 1})
    client[db_name].foo.drop()
    if client[db_name].collection_names():
        raise ValueError("database has collections")
    primary = client['config'].databases.find_one(db_name)['primary']
    if primary != shard:
        res = client.admin.command(
            'movePrimary', value=db_name, to=shard)
        if not res.get('ok'):
            raise RuntimeError(str(res))
    return (
        f"Successfully created {db_name} in {shard} via {client.nodes} "
        f"from {hostname}")