定语从句there was to do剖析

I took my cue in a moment, and did the only rational thing there was to do.

I reached the note towards him, and said, carelessly: Give me the change, please.

这里的and did the only rational thing there was to do. 中：there was to do 不知道。

若是there was， 那是定语从句。

there was to do 是不是也是定语从句？若是，如何理解呢？do代表了什么呢？

there do 麻烦老师剖析一下，谢谢！

the only rational thing （that）there was to do.

省略的that指代先行词，并在定语从句作主语，to do为that的定语，动宾关系。比较：

there was one thing to do 不定式为定语，表示动宾关系。