Array primary keys (e.g. composite) break `collection_singular_ids` method when using preload about rails HOT 2 OPEN

In this case, reflection.association_primary_key returns ["id"], which, when passed to the pluck method, is further nested into [["id"]] due to the splat operator (*keys) in the method's parameters. Consequently, this nesting results in keys.first within the pluck method being ["id"], rather than the expected straightforward "id".

When using the Enumerable#pluck method with an array argument, like [:id, :post_id], it leads to unexpected behavior. The method interprets the array as a single, composite attribute, which doesn't match any keys in the hash, resulting in nil values for each element:

[{ id: 1, post_id: 1 }, { id: 2, post_id: 1 }, { id: 3, post_id: 1 }].pluck([:id, :post_id])
# Output: [nil, nil, nil]

To accurately extract multiple attributes, each attribute name should be passed as a separate argument. This allows the method to correctly identify and retrieve the values for each attribute from the hashes:

[{ id: 1, post_id: 1 }, { id: 2, post_id: 1 }, { id: 3, post_id: 1 }].pluck(:id, :post_id)
# Output: [[1, 1], [2, 1], [3, 1]]

# This works too
[{ id: 1, post_id: 1 }, { id: 2, post_id: 1 }, { id: 3, post_id: 1 }].pluck(*[:id, :post_id])
# Output: [[1, 1], [2, 1], [3, 1]] 

We can utilize the splat operator (*) to distribute the array's elements as separate arguments to the method.

target.pluck(*reflection.association_primary_key)

from rails.