๐ 4 key methods
- joins
- eager_load
- preload
- includes
๐ joins
integrate by INNER JOIN
- doesn't cache association, so if you don't need data which is created again, you should use it.
- save memory allocated spaces because ActiveRecord object doesn't cache.
Skill.joins(:skill_category).limit(5)
โ
SELECT "skills".*
FROM "skills"
INNER JOIN "skill_categories"
ON "skill_categories"."id" = "skills"."skill_category_id"
LIMIT ? [["LIMIT", 5]]
๐ eager_load
integrate by LEFT OUTER JOIN with cache
- faster than preload() because it creates only one SQL
- it can use WHERE in table which is integrated by JOIN (preload() can't do it)
Skill.eager_load(:skill_category).limit(5)
โ
SELECT "skills"."id" AS t0_r0, "skills"."name" AS t0_r1, "skills"."user_id" AS t0_r2, "skills"."skill_category_id" AS t0_r3, "skills"."created_at" AS t0_r4, "skills"."updated_at" AS t0_r5, "skill_categories"."id" AS t1_r0, "skill_categories"."name" AS t1_r1, "skill_categories"."reccomend" AS t1_r2, "skill_categories"."created_at" AS t1_r3, "skill_categories"."updated_at" AS t1_r4
FROM "skills"
LEFT OUTER JOIN "skill_categories"
ON "skill_categories"."id" = "skills"."skill_category_id"
LIMIT ? [["LIMIT", 5]]
๐ preload
use multiple SQLs with cache
- recommend to use it when you have big table which you don't wanna JOIN
- * it's impossible to use WHERE because it is not integrated by JOIN
Skill.preload(:skill_category).limit(5)
โ
# this one
SELECT "skills".* FROM "skills" LIMIT ? [["LIMIT", 5]]
# and this one
SELECT "skill_categories".*
FROM "skill_categories"
WHERE "skill_categories"."id"
IN (?, ?, ?, ?, ?) [[nil, 483], [nil, 583], [nil, 901], [nil, 181], [nil, 147]]
๐ includes
if you use where, join, references method at least one, executes as eager_load, otherwise preload
# just includes
Skill.includes(:skill_category).limit(5)
โ
# this one
SELECT "skills".* FROM "skills" LIMIT ? [["LIMIT", 5]]
# and this one
SELECT "skill_categories".*
FROM "skill_categories"
WHERE "skill_categories"."id"
IN (?, ?, ?, ?, ?) [[nil, 483], [nil, 583], [nil, 901], [nil, 181], [nil, 147]]
# using where()
Skill.includes(:skill_category).where(skill_categories: { name: 'baseball' })
โ
# just one SQL like eager_load ๐
SELECT "skills"."id" AS t0_r0, "skills"."name" AS t0_r1, "skills"."user_id" AS t0_r2, "skills"."skill_category_id" AS t0_r3, "skills"."created_at" AS t0_r4, "skills"."updated_at" AS t0_r5, "skill_categories"."id" AS t1_r0, "skill_categories"."name" AS t1_r1, "skill_categories"."reccomend" AS t1_r2, "skill_categories"."created_at" AS t1_r3, "skill_categories"."updated_at" AS t1_r4
FROM "skills"
LEFT OUTER JOIN "skill_categories"
ON "skill_categories"."id" = "skills"."skill_category_id"
WHERE "skill_categories"."name" = ? [["name", "baseball"]]
includes method is convenience as you see, but when other developer will see includes method, they need to think it means "preload" or "eager_load"
Top comments (1)
Great breakdown of these different approaches.
My memories a bit rusty, but is an ActiveRecord object instantiated with a
:joinsnot "save"-able? As in I can't successfully call:updateon that instance.