constructor({ modelClass }) {
    this.rootModelClass = modelClass;

    // The relation expression to join.
    this.expression = null;

    // Explicit modifiers for the relation expression.
    this.modifiers = null;

    this.options = defaultOptions();
    this.tableTree = null;
    this.internalSelections = null;
  }
Given a relation expression, builds a query using joins to fetch it.