def migrate_database(adapter):
    """Migrate an old loqusdb instance to 1.0
    
    Args:
        adapter
    
    Returns:
        nr_updated(int): Number of variants that where updated
    """
    
    all_variants = adapter.get_variants()
    nr_variants = all_variants.count()
    nr_updated = 0
    with progressbar(all_variants, label="Updating variants", length=nr_variants) as bar:
        for variant in bar:
            # Do not update if the variants have the correct format
            if 'chrom' in variant:
                continue
            nr_updated += 1
            splitted_id = variant['_id'].split('_')
            
            chrom = splitted_id[0]
            start = int(splitted_id[1])
            ref = splitted_id[2]
            alt = splitted_id[3]
            
            # Calculate end
            end = start + (max(len(ref), len(alt)) - 1)
            
            adapter.db.variant.find_one_and_update(
                {'_id': variant['_id']},
                {
                    '$set': {
                        'chrom': chrom,
                        'start': start,
                        'end': end
                    }
                }
            )
    
    return nr_updated