Add ActiveRecord::Relation#extract_associated for extracting associated record

[dhh]

It's sometimes more convenient to access a certain set of records by going through a scoped relation and its associations. It's easy enough to do this today using #preload and #collect, but it's repetitive and not descriptive of the intent.

This is where #extract_associated comes in. Like so:

account.memberships.extract_associated(:user)

Which is short-hand for and describing the intent of:

account.memberships.preload(:user).collect(&:user)

[kaspth]

Perhaps we should wrap the extract in an assert_queries 1, such that we test the preload query optimization.

[dhh]

Good idea!

[simi]

🤔 what about load_associated?

[dhh]

Load associated is the same as preload. The important part here is that the records returned are those of the association, not of the relation. Extract is the word for taking something like that.

…

On Mar 28, 2019, at 16:41, Josef Šimánek ***@***.***> wrote: 🤔 what about load_associated? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub, or mute the thread.

[simi]

I'm just trying to find out something similar in current method names since extract is not common term in ActiveRecord terminology. There's already load method and that was origin of my initial idea.

I understand this as a pluck of association on relation. My second idea was pluck_associated, but it sounds a little strange me. 🤔

[rafbm]

As it is, I feel extract_associated brings little gain, especially as it only “supports” belongs_to and has_one associations. It would be a much more meaningful addition if it also supported has_many, ie. changing collect to flat_map under the hood.

I actually think the current behavior can be quite confusing. As the doc states, extract_associated returns a “collection of X records”:

account.memberships.extract_associated(:user)
# => Returns collection of User records

You could easily expect the following to return a “collection of Comment records”:

category.posts.extract_associated(:comments)

…but you’d be wrong, as this will instead return a “collection of Comment collections”.