forEachRelation (mapper, opts, fn, thisArg) {
    const relationList = mapper.relationList || []
    if (!relationList.length) {
      return
    }
    relationList.forEach(function (def) {
      utils._forRelation(opts, def, fn, thisArg)
    })
  }
Recursively iterate over a {@link Mapper}'s relations according to
`opts.with`.

@method utils.forEachRelation
@param {Mapper} mapper Mapper.
@param {object} opts Configuration options.
@param {Function} fn Callback function.
@param {*} thisArg Execution context for the callback function.
@since 3.0.0