How to use Includes in Rails

How to use Includes in rails

Includes is a handy method to do eager loading on your associations as stated in the rails documentation - "With includes, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries."

Lets look at some of the ways to use includes

Just plain includes statement

users = User.includes(:url_shortner_logs)

User Load (0.8ms)  SELECT "users".* FROM "users"
UrlShortnerLog Load (1.2ms)  SELECT "url_shortner_logs".* FROM "url_shortner_logs" WHERE "url_shortner_logs"."user_id" IN (2, 3, 4, 1)

It fires two queries and loads User and UrlShortnerLog tables in memory.

users.first.url_shortner_logs

The above command does not make a DB call instead loads the data from memory

Includes with where

UrlShortner.includes(:url_shortner_logs).where(url_shortner_logs: {created_at: Date.today})

SQL (1.2ms)  SELECT "url_shortners"."id" AS t0_r0, "url_shortners"."short_url" AS t0_r1, "url_shortners"."original_url" AS t0_r2, "url_shortners"."sanitized_url" AS t0_r3, "url_shortners"."created_at" AS t0_r4, "url_shortners"."updated_at" AS t0_r5, "url_shortner_logs"."id" AS t1_r0, "url_shortner_logs"."url_shortner_id" AS t1_r1, "url_shortner_logs"."user_id" AS t1_r2, "url_shortner_logs"."browser" AS t1_r3, "url_shortner_logs"."version" AS t1_r4, "url_shortner_logs"."platform" AS t1_r5, "url_shortner_logs"."created_at" AS t1_r6, "url_shortner_logs"."updated_at" AS t1_r7 FROM "url_shortners" LEFT OUTER JOIN "url_shortner_logs" ON "url_shortner_logs"."url_shortner_id" = "url_shortners"."id" WHERE "url_shortner_logs"."created_at" = $1  [["created_at", "2017-07-30"]]

The above query makes a single DB call and does left outer join on the where conditions.

Includes with where and references

UrlShortner.includes(:url_shortner_logs).where("url_shortner_logs.created_at= '2017–07–30'")

This query will throw ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR. It does not have reference to the table UrlShortnerLogs To solve this, rails provides a method called references and it works only in conjunction with includes

UrlShortner.includes(:url_shortner_logs).where("url_shortner_logs.created_at= '2017-07-30'").references(:url_shortner_logs)

SQL (0.5ms)  SELECT "url_shortners"."id" AS t0_r0, "url_shortners"."short_url" AS t0_r1, "url_shortners"."original_url" AS t0_r2, "url_shortners"."sanitized_url" AS t0_r3, "url_shortners"."created_at" AS t0_r4, "url_shortners"."updated_at" AS t0_r5, "url_shortner_logs"."id" AS t1_r0, "url_shortner_logs"."url_shortner_id" AS t1_r1, "url_shortner_logs"."user_id" AS t1_r2, "url_shortner_logs"."browser" AS t1_r3, "url_shortner_logs"."version" AS t1_r4, "url_shortner_logs"."platform" AS t1_r5, "url_shortner_logs"."created_at" AS t1_r6, "url_shortner_logs"."updated_at" AS t1_r7 FROM "url_shortners" LEFT OUTER JOIN "url_shortner_logs" ON "url_shortner_logs"."url_shortner_id" = "url_shortners"."id" WHERE (url_shortner_logs.created_at= '2017-07-30')

We can also observe that includes makes two DB calls in basic query condition and makes a single DB call when where clause is being used.

How to use Includes in rails

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