Support using a nil relation as a condition

[ghiculescu]

I want to define an ability check that only applies if a has_many relation returns an empty array. The use case is I want to only allow deleting a record if it has no associated records for a given association.

In ActiveRecord you can do this with syntax like this:

Article.joins(:comments).where(comments: { id: nil })

This generates the SQL:

SELECT "articles".* FROM "articles" INNER JOIN "comments" ON "comments"."article_id" = "articles"."id" WHERE "comments"."id" IS NULL

Which returns an empty array.

This PR makes cancan support the same style of query. So you can do:

@ability.can :destroy, Article, comments: { id: nil }

This will allow you to :destroy an Article that has no comments, but not an article that has comments.

[coorasse]

I have one or two questions and maybe a request for an additional test. good job!

[coorasse]

why does it even try? 😄

[ghiculescu]

Haha, good question. I tried to avoid adding too much logic specific to this scenario, and mostly rely on how the Rails querying interface already works. In this case, we are generating a query that can never be true, but Rails hasn't necessarily understood that or optimised it out.

[ghiculescu]

I suppose I could try and refactor it to return Article.none in this case, but it feels like it could add a fair bit more maintenance load.

[coorasse]

would can :read, Article, comments: nil work as well? I'd expect so from 3.2.0.

And what about can :read, Article, comments: []? Maybe I'd expect this to work as well as can :read, Article, comments: { id: [] }

[ghiculescu]

Not currently. The cancan conditions need to match conditions you'd pass into AR#where. Both of your examples wouldn't work as regular queries.

(byebug) Article.left_outer_joins(:comments).where(comments: nil)
#<ActiveRecord::Relation []>
(byebug) Article.left_outer_joins(:comments).where(comments: nil).to_sql
"SELECT \"articles\".* FROM \"articles\" LEFT OUTER JOIN \"comments\" ON \"comments\".\"article_id\" = \"articles\".\"id\" WHERE \"articles\".\"id\" IS NULL" # note how it looks for article.ids IS NULL, not comments.id

(byebug) Article.left_outer_joins(:comments).where(comments: [])
#<ActiveRecord::Relation []>
(byebug) Article.left_outer_joins(:comments).where(comments: []).to_sql
"SELECT \"articles\".* FROM \"articles\" LEFT OUTER JOIN \"comments\" ON \"comments\".\"article_id\" = \"articles\".\"id\" WHERE 1=0"

Whereas with my example:

(byebug) Article.left_outer_joins(:comments).where(comments: { id: nil })
#<ActiveRecord::Relation [#<Article id: 1, name: nil, created_at: "2020-12-14 00:50:15", updated_at: "2020-12-14 00:50:15", published: nil, secret: nil, priority: nil, category_id: nil, project_id: nil, user_id: nil>]>
(byebug) Article.left_outer_joins(:comments).where(comments: { id: nil }).to_sql
"SELECT \"articles\".* FROM \"articles\" LEFT OUTER JOIN \"comments\" ON \"comments\".\"article_id\" = \"articles\".\"id\" WHERE \"comments\".\"id\" IS NULL"

[ghiculescu]

I'm open to trying to support more use cases, but I think it'd add a bit more complexity as we'd need to translate specific values (nils, [], etc) into the corresponding AR query statements.

[ghiculescu]

@coorasse anything more you need from me to get this merged?

[coorasse]

I believe this is ready to be merged. Could you squash and rebase, please?

[ghiculescu]

I'm just gonna run our CI suite against this now that a few changes upstream have been merged in.

[coorasse]

Sure! Just let me know, but I believe is good to go 👍

[ghiculescu]

@coorasse green build. Go for it.