Querying the Database

Let's dive a little deeper into ActiveRecord and see how we can find and search for records:
### Lookups ###

# find the "first" `Post`
# notice the ORDER BY and LIMIT in the generated SQL
Post.first
#  Post Load (0.1ms)  SELECT  "posts".* FROM "posts"  ORDER BY "posts"."id" ASC LIMIT 1
#=> #<Post:0x000000059f3378
# id: 1,
# title: "Blog 1",
# body: "Lorem ipsum dolor sit amet.",
# author: "Kevin",
# created_at: Sun, 07 Dec 2014 00:00:00 UTC +00:00,
# updated_at: Sun, 07 Dec 2014 00:00:00 UTC +00:00>

# some similar methods...

Post.second
#  Post Load (0.1ms)  SELECT  "posts".* FROM "posts"  ORDER BY "posts"."id" ASC LIMIT 1 OFFSET 1
#=> #<Post:0x00000005a8b100 id: 2, ...

Post.last
#  Post Load (0.4ms)  SELECT  "posts".* FROM "posts"  ORDER BY "posts"."id" DESC LIMIT 1
#=> #<Post:0x00000005ae99f8 id: 3, ...

# we can even get counts
Post.count
#  (0.1ms)  SELECT COUNT(*) FROM "posts"
# => 3


# at this point we're familiar with the idea of `.find`
Post.find 1
#  Post Load (0.1ms)  SELECT  "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT 1  [["id", 1]]
#=> #<Post:0x00000005b86f50 id: 1, ...

# but with Active Record, we can also look up by several ids at once
Post.find [1,2]
#  Post Load (0.4ms)  SELECT "posts".* FROM "posts" WHERE "posts"."id" IN (1, 2)
#=> [#<Post:0x00000005bd8558 id: 1, ...>,
# #<Post:0x00000005bd83f0 id: 2, ...>]


# we're also familiar with `.where`
Post.where(author: 'Brad')
#  Post Load (0.3ms)  SELECT "posts".* FROM "posts" WHERE "posts"."author" = ?  [["author", "Brad"]]
#=> [#<Post:0x0000000207b528 ... author: "Brad", ...

# but we can also use special finders like this
Post.find_by_author 'Brad'
#  Post Load (0.1ms)  SELECT  "posts".* FROM "posts" WHERE "posts"."author" = ? LIMIT 1  [["author", "Brad"]]
#=> #<Post:0x00000005c6ee68 ... author: "Brad", ...

### ActiveRecord_Relation ###

# it's important to note what `.where` really gives us
Post.where(author: 'Brad').class
# => Post::ActiveRecord_Relation

# this ActiveRecord_Relation lets us chain calls together

# for example, if we wanted, for some reason,
# the first three comments ordered by author,
# we could chain `.author` and `.limit` calls
Comment.order(:author).limit(3)
#  Comment Load (0.2ms)  SELECT  "comments".* FROM "comments"  ORDER BY "comments"."author" ASC LIMIT 3
# => [#<Comment:0x00000005f84f58
#   id: 2,
#   body: "Lorem ipsum dolor sit amet.",
#   author: "DHH",
#   post_id: 1,
#   created_at: Thu, 11 Dec 2014 00:00:00 UTC +00:00,
#   updated_at: Thu, 11 Dec 2014 00:00:00 UTC +00:00>,
#  #<Comment:0x00000005f84df0
#   id: 5,
#   body: "Lorem ipsum dolor sit amet.",
#   author: "DHH",
#   post_id: 2,
#   created_at: Sun, 14 Dec 2014 00:00:00 UTC +00:00,
#   updated_at: Sun, 14 Dec 2014 00:00:00 UTC +00:00>,
#  #<Comment:0x00000005f84c88
#   id: 7,
#   body: "Lorem ipsum dolor sit amet.",
#   author: "DHH",
#   post_id: 3,
#   created_at: Tue, 16 Dec 2014 00:00:00 UTC +00:00,
#   updated_at: Tue, 16 Dec 2014 00:00:00 UTC +00:00>]

# this is possible because each of these methods is...
Comment.order(:author).limit(3).class
# => Comment::ActiveRecord_Relation

# but when we also iterate over these objects as if they were arrays
Comment.order(:author).limit(3).map {|comment|
  [comment.author, comment.created_at]
}
#  Comment Load (0.9ms)  SELECT  "comments".* FROM "comments"  ORDER BY "comments"."author" ASC LIMIT 3
#=> [["DHH", Thu, 11 Dec 2014 00:00:00 UTC +00:00],
# ["DHH", Sun, 14 Dec 2014 00:00:00 UTC +00:00],
# ["DHH", Tue, 16 Dec 2014 00:00:00 UTC +00:00]]

# the chaining of these calls basically builds up a query
# and the SQL is generated and executed upon something like
# one of the following methods being called on the object:
#
# .first
# .last
# .all
# .each
#
# which is why our `.map` above works as we'd expect it to
For more resources on ActiveRecord models, validations, associations, and more: