Meaning of “Aotagai(青田買い)”

Basic meaning of “Aotagai(青田買い)” is “buy rice before it is reaped”.
Otherwise, meaning of it as a kind of idiom is “recruit college students for future jobs long before they graduate”.
Additionally, these days, we use this idiom in other meanings.
For example, if someone start supporting a idol group before the group become popular, some people might call it “Aotagai(青田買い)”.
So “someone start supporting or buying someone/something before it become popular”, I might call it “Aotagai(青田買い)”.

Example Conversation

A: Who is your oshi(推し), these days?
B: I think you don’t know her name, she is not popular at all.
A: Oh, you like “aotagai(青田買い)”, I knew it.

A: 最近の推しは？
B: 言っても分からないと思うよ、全然メジャーじゃないから。
A: そっか、お前は青田買い好きだもんな