Get a wolfram entity value attribute at a time

Question

This is something I've always struggled with. Let's say I want to know the population of Mexico City, I can easily do

Mexico City ["Population"]

But say I'm making a chart over time and I want to get the population of Mexico City in 1800AD. I understand that not all data is going to be available, but just syntactically, how do I query for that?

Answer 1

I was pointed to this question which provided the hint I needed.

Seems like confusingly, historical info might not be stored on the proper name entities at all. Instead I had to invoke the CityData function

CityData[Mexico City, {"Population, All}]

Why this dataset sucks so much is a separate question...