If you describe an action, event, or situation as an object lesson, you think that it demonstrates the correct way to do something, or that it demonstrates the truth of a particular principle.
It was an object lesson in how to use television as a means of persuasion. [+ on/in]
object lesson in British English
noun
1.
a convincing demonstration of some principle or ideal
2.
(esp formerly) a lesson in which a material object forms the basis of the teaching and is available to be inspected
object lesson in American English
an actual or practical demonstration or exemplification of some principle
Examples of 'object lesson' in a sentence
object lesson
What we ended up with was an object lesson in how important good service is.
Times, Sunday Times (2009)
While perhaps not as trendy, a good old-fashioned object lesson still has power.