Function QueryCursor

lib/cursor/QueryCursor.js:37–93

* A QueryCursor is a concurrency primitive for processing query results * one document at a time. A QueryCursor fulfills the Node.js streams3 API, * in addition to several other mechanisms for loading documents from MongoDB * one at a time. * * QueryCursors execute the model's pre `find` hooks

(query)

 */

function QueryCursor(query) {
  // set autoDestroy=true because on node 12 it's by default false
  // gh-10902 need autoDestroy to destroy correctly and emit 'close' event
  Readable.call(this, { autoDestroy: true, objectMode: true });

  this.cursor = null;
  this.skipped = false;
  this.query = query;
  const model = query.model;
  this._mongooseOptions = {};
  this._transforms = [];
  this.model = model;
  this.options = {};
  model.hooks.execPre('find', query, (err) => {
    if (err != null) {
      if (err instanceof kareem.skipWrappedFunction) {
        const resultValue = err.args[0];
        if (resultValue != null && (!Array.isArray(resultValue) || resultValue.length)) {
          const err = new MongooseError(
            'Cannot `skipMiddlewareFunction()` with a value when using ' +
            '`.find().cursor()`, value must be nullish or empty array, got "' +
            util.inspect(resultValue) +
            '".'
          );
          this._markError(err);
          this.listeners('error').length > 0 && this.emit('error', err);
          return;
        }
        this.skipped = true;
        this.emit('cursor', null);
        return;
      }
      this._markError(err);
      this.listeners('error').length > 0 && this.emit('error', err);
      return;
    }
    Object.assign(this.options, query._optionsForExec());
    this._transforms = this._transforms.concat(query._transforms.slice());
    if (this.options.transform) {
      this._transforms.push(this.options.transform);
    }
    // Re: gh-8039, you need to set the `cursor.batchSize` option, top-level
    // `batchSize` option doesn't work.
    if (this.options.batchSize) {
      // Max out the number of documents we'll populate in parallel at 5000.
      this.options._populateBatchSize = Math.min(this.options.batchSize, 5000);
    }

    if (model.collection._shouldBufferCommands() && model.collection.buffer) {
      model.collection.queue.push([
        () => _getRawCursor(query, this)
      ]);
    } else {
      _getRawCursor(query, this);
    }
  });
}