Calling pluck is instantly making a SELECT, while passing the relationship allows rails to build a correct query.
Before (2 selects):
```
pry(main)> Post.where(topic_id: Topic.where(id: [1,3,4]).pluck(:id)).count
(1.3ms) SELECT "topics"."id" FROM "topics" WHERE "topics"."deleted_at" IS NULL AND "topics"."id" IN (1, 3, 4)
Post Count (0.5ms) SELECT COUNT(*) FROM "posts" WHERE "posts"."deleted_at" IS NULL AND "posts"."topic_id" IN (1, 3, 4)
```
After (1 select):
```
pry(main)> Post.where(topic_id: Topic.where(id: [1,3,4])).count
Post Count (2.7ms) SELECT COUNT(*) FROM "posts" WHERE "posts"."deleted_at" IS NULL AND "posts"."topic_id" IN (SELECT "topics"."id" FROM "topics" WHERE "topics"."deleted_at" IS NULL AND "topics"."id" IN (1, 3, 4))
```
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