const Asset = new warehouse.Schema<AssetSchema>({
    _id: {type: String, required: true},
    path: {type: String, required: true},
    modified: {type: Boolean, default: true},
    renderable: {type: Boolean, default: true}
  });

  Asset.virtual('source').get(function() {
    return join(ctx.base_dir, this._id);
  });

const Cache = new warehouse.Schema<CacheSchema>({
    _id: {type: String, required: true},
    hash: {type: String, default: ''},
    modified: {type: Number, default: Date.now() } // UnixTime
  });

  Cache.static('compareFile', function(id: string,
    hashFn: (id: string) => Promise<string>,
    statFn: (id: string) => Promise<fs.Stats>): Promise<{ type: string }> {
    const cache = this.findById(id) as Document<CacheSchema>;

    // If cache does not exist, then it must be a new file. We have to get both
    // ...
  });