Rails Active Record select parent and child as one result

Question

I have a parent / child relationship in my application ``` class Polling has_many :alerts, :dependent => :destroy class Alert belongs_to :polling ``` On my index page for the alerts, I need to show some data from each parent, and this results in two queries ``` Alert Load (6.1ms) SELECT * FROM (SELECT * FROM "ALERTS" INNER JOIN "POLLINGS" ON "POLLINGS"."ID" = "ALERTS"."POLLING_ID" ORDER BY "ALERTS"."ID" DESC) WHERE ROWNUM <= 1 Polling Load (1.8ms) SELECT "POLLINGS".* FROM "POLLINGS" WHERE "POLLINGS"."ID" = 10113 AND ROWNUM <= 1 ``` Obviously, this makes the page loading time quite horrendous as it has to loop through each one and pull the parent object as well. I have tried a few things, such as ``` > Alert.joins(:polling).where(...) > Alert.includes(:polling).where(...) > Alert.joins(:polling).select('*').where(...) ``` And each time I get two distinct queries when I visit my index page. One for each Alert, and then another to get its parent data. How can I do this on one line so tha…

Accepted Answer

If you want to specify SQL in detail, you can always try `find` method passing :include, :joins, :where etc. `includes` or `joins` does same thing down the line when you try to access to data but behaviour of activerecord is different. For example, following 3 ways set up object array but they are different.

[1]

```
alerts = Alert.joins(:polling)
```

[2]

```
alerts = Alert.includes(:polling)
```

[3]

```
alerts = Alert.find(:all, :includes => :polling, :joins => :polling)
```

when you do

```
alerts.each do |alert|
  alert.pollings.each do |polling|
    p polling
  end
end
```

you can see how activerecord tries to get data from DB or cache. The [3] way, I believe, throws only one query when others throw multiple queries.

Rails Active Record select parent and child as one result

Solution

Problem