At the very least on PostgreSQL (I only say that because that's where
I'm the most familiar) there's no reason to need to cast based purely
on the class's attributes -- the database result knows the database
types of the columns returns. For example, my relation_to_struct gem
does this for arbitrary calculated columns using only the ActiveRecord
APIs:
https://github.com/jcoleman/relation_to_struct/blob/master/lib/relation_to_struct/active_record_relation_extension.rb#L26
I believe `pluck` should guarantee the same.
On Sat, Apr 20, 2019 at 1:28 AM Stan Lo <stan001...@gmail.com> wrote:
>
> From the examples in documents, it looks like AR#pluck is mainly for
> retrieving columns from the same table. Like
> Post.pluck(:author_id)
>
> But we can also find some people using pluck for joined tables' columns, like
> Post.joins(:author).pluck("author.name", :title)
>
> And normally this works fine, except for cases like
> https://github.com/rails/rails/issues/36042.
>
> class Post < ActiveRecord::Base
> enum status: {
> a: "a",
> b: "b",
> c: "c"
> }
> has_many :comments
> end
>
> class Comment < ActiveRecord::Base
> enum status: {
> red: "red",
> green: "green",
> blue: "blue"
> }
> belongs_to :post
> end
>
> class BugTest < Minitest::Test
> def test_association_stuff
> post = Post.create!(status: "a")
> post.comments << Comment.create!(status: "blue", not_enum: "blue")
>
> expected = [["a", "blue", "blue"]]
> actual = Post.includes(:comments).pluck(:status, "comments.status",
> "comments.not_enum")
> assert_equal expected, actual
> end
> end
>
>
> After digging into the issue, I think plucking joined tables' columns is kind
> of a misuse of pluck. Here's the source code of pluck
>
> def pluck(*column_names)
> if loaded? && (column_names.map(&:to_s) - @klass.attribute_names -
> @klass.attribute_aliases.keys).empty?
> return records.pluck(*column_names)
> end
>
> if has_include?(column_names.first)
> relation = apply_join_dependency
> relation.pluck(*column_names)
> else
> klass.disallow_raw_sql!(column_names)
> relation = spawn
> relation.select_values = column_names
> result = skip_query_cache_if_necessary {
> klass.connection.select_all(relation.arel, nil) }
> result.cast_values(klass.attribute_types)
> end
> end
>
>
> And the cause of the issue above is
>
> result.cast_values(klass.attribute_types)
>
> Pluck seems assume it'll only be used for the same table, so it uses the
> original class' attribute types to cast column values (in this case, the
> Post's attribute types). Hence when the joined table's columns need different
> casting methods (like different enum definitions), Rails may get confused.
> (For generic types like integer, string that won't be a problem though,
> that's why most people don't have issue). And for our issue example, Rails
> tried to use Post's status type definition to cast Comment's status type, and
> as expected it failed.
>
> So my question is, should we have a full support on this behavior? Which
> means we'd need to be able to cast types for every tables envolved. And this
> won't be a small change since we now need to get all tables' type
> definitions. Or we should consider this as a misuse and not receommending
> using it (or even deprecate it).
>
>
>
>
>
>
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