code: function extractPaths(schemaPaths, model) {
    return reduce(schemaPaths, (fields, schemaPath) => (
      merge(fields, extractPath(schemaPath, model))
    ), {});
  }
docs: Merge sub-document tree chunks
  @method extractPaths
  @param {Object} schemaPaths
  @param {Object} model
  @return {Object) extractedSchemaPaths