toJSON (opts) {
    return this.mapCall('toJSON', opts)
  }
Return the plain JSON representation of all items in this collection.
Assumes records in this collection have a toJSON method.

@method Collection#toJSON
@since 3.0.0
@param {object} [opts] Configuration options.
@param {string[]} [opts.with] Array of relation names or relation fields
to include in the representation.
@returns {Array} The records.