Rails ActiveRecord Behaviour?

Question

I wanted to understand how ActiveRecord works in the following scenario.

@messages = Message.joins(:users).where("some condition").uniq

now if i use @messages.sum(:url_view_count) I see the query is interpreted again as

SELECT DISTINCT SUM ("messages"."url_view_count")  FROM "messages" INNER JOIN "users" ON .. and conditons

why the whole query starts again? Can it do sum among the filtered out @messages right?

And, why the interpreted query is `DISTINCT SUM(url_view_count)'?

Doesn't this mess up my result?

If the column url_view_count has 1, 1, 2. I am expecting 1+1+2 = 4, But this query gives me result as 1+2 = 3.

Please help me understand this.

EJAg · Accepted Answer · 2017-09-25 05:48:56Z

1

@messages is an ActiveRecord::Relation. It's lazy evaluated. So @messages does not execute any SQL commands until it has too. In this case, when you call sum on it.
You have DISTINCT because you chained #uniq there. But I don't think it matters here, because it modifies SUM here (the SQL here selects SUM, not messages).

edited Sep 25, 2017 at 5:48

answered Sep 25, 2017 at 5:41

EJAg

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2 Comments

Sachin srinivasan Over a year ago

Message.pluck(:url_view_count).compact.count , i get 46. Message.pluck(:url_view_count).uniq.compact.count i get 10. so we understand many of values are repeated. Now the .uniq.sum(:url_view_count) from query in question is ignoring repeated values.

EJAg Over a year ago

#pluck returns Array, and the #uniq here is an array method, but that in your question is an ActiveRecord::Relation method. You'll see that these are very different queries.

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