// removes all document metadata and also removes the file, in case
    // no other documents has the same file attached to it
    function removeDocumentMetadatas(docIDs, cb){
        async.each(docIDs, (docID, callback) => {
            DocumentMetadata.findAll({
                where:{
                    document_id: docID
                },
                attributes: ['filehash', 'effective_path']
            }).then((metas) => {
                // now go over each filehash and check if other
                // metadata records exist with the same file
                // if there are not such records, we can delete the document
                async.each(metas, (meta, filecb) => {
                    var filehash = meta.filehash;
                    var filepath = meta.effective_path;
                    DocumentMetadata.findAll({
                        where: {
                            filehash: filehash
                        },
                        attributes: ['document_id']
                    }).then((metasWithSameHash) => {
                        // check all fetched meta records
                        var a = true;
                        _.each(metasWithSameHash, (meta) => {
                            if(meta.document_id !== docID){
                                a = false;
                            }
                        });
                        // if a false no other document has the same file
                        if(!a){
                            // delete file
                            fs.unlink(filepath, (err) => {
                                if(err){
                                    filecb(err);
                                }
                                filecb();
                            });
                        }else{
                            filecb();
                        }
                    });
                }, (err) => {
                    if(err){
                        callback(err);
                        return;
                    }
                    // finally delete all document metadatas
                    DocumentMetadata.destroy({
                        where: {
                            document_id: docID
                        }
                    }).then(() => {
                        callback();
                    }).catch((err) => callback(err));
                });
            });
        }, (err) => {
            if(err){
                cb(err, false);
                return;
            }
            cb(null, docIDs);
        });
    }