Include a preload joins vs. no joins discussion in the preload docs #4517
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PragTob
commented
Oct 2, 2024
lib/ecto/query.ex
Outdated
| @@ -2667,6 +2667,34 @@ defmodule Ecto.Query do | |||
| where: l.inserted_at > c.updated_at, | |||
| preload: [:author, comments: {c, likes: l}] | |||
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| ## Choosing Between Preloading with Joins vs. Separate Queries | |||
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I wasn't sure whether to make it a separate section with a heading or not, but I ended up doing it because it got too big to just be part of the normal text.
I also put it "first" as it seemed most relevant to the preceeding setion flow-wise. Happy to re-evaluate and change!
I'm not sure if Ecto wants to provide this level of guidance in the docs, so I understand if this PR won't be merged. I've just seen many well-meaning people do this wrong (imo) by defaulting to joining all associations just to preload them, resulting in a lot more code and potentially worse performance. I also vaguely remember a tweet from José calling this out, but couldn't find it again. Here I tried to summarize what I think I know about the topic, hoping that it's accurate and helpful.
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Updated, removed section about database load. Thanks for the feedback everyone! |
josevalim
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Oct 7, 2024
greg-rychlewski
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Oct 7, 2024
josevalim
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💚 💙 💜 💛 ❤️ |
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I'm not sure if Ecto wants to provide this level of guidance in the docs, so I understand if this PR won't be merged.
I've just seen many well-meaning people do this wrong (imo) by defaulting to joining all associations just to preload them, resulting in a lot more code and potentially worse performance.
I also vaguely remember a tweet from José calling this out, but couldn't find it again.
Here I tried to summarize what I think I know about the topic, hoping that it's accurate and helpful.
As usual, thank you all very much for your work! 💚