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Ruby on Rails 5 Basics

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Rails Console: Reading Model Objects

1:57 with Jay McGavren

Since reading from the database is easiest, we'll go a little bit out of order in our tour of CRUD operations, and start with "read" operations.

We can get a collection of all of the "Post" objects in our database by calling the "all" method on the "Post" class. We can type "Post.all" at the prompt...

  • We'll see this SQL query: "Post Load (0.2ms) SELECT "posts".* FROM "posts"", just like we saw when "Post.all" was called from our posts controller earlier.
  • Then, the Rails console displays the return value of that method call, the collection of all "Post" objects: => #<ActiveRecord::Relation [#<Post id: 1, title: "Hello, Rails!", created_at: "2016-05-28 18:21:19", updated_at: "2016-05-28 18:21:19">, #<Post id: 3, title: "Second post", created_at: "2016-05-28 23:04:35", updated_at: "2016-05-28 23:04:35">]>

We can get back just the most recent "Post" object by calling the "last" method on the "Post" class. If we type "Post.last" at the prompt...

  • We'll see the same SQL query that was generated for "Post.all", but limited to just one record: "Post Load (0.1ms) SELECT "posts".* FROM "posts" ORDER BY "posts"."id" DESC LIMIT ? [["LIMIT", 1]]".
  • The return value consists of just one "Post" model object: #<Post id: 3, title: "Second post", created_at: "2016-05-28 23:04:35", updated_at: "2016-05-28 23:04:35">

Notice that each model object has had a value automatically assigned to its "id" database column. We can pass that ID to the "find" class method to load a particular record from the database.

  • The last "Post" object here has an ID of 3. We can type "Post.find(3)"...
  • We'll see the SQL query SELECT "posts".* FROM "posts" WHERE "posts"."id" = ? LIMIT ? [["id", 1], ["LIMIT", 1]]. This query is limited to just one database row, the one with an ID matching the ID we specified.
  • The return value again consists of just one "Post" object, the one whose ID matched: #<Post id: [??], title: "Hello, Rails!", created_at: "2016-07-07 22:53:46", updated_at: "2016-07-07 22:53:46">

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    Since reading from the database is easiest, we'll go a little bit 0:00
    out of order in our tour of CRUD operations and start with read operations. 0:03
    We can get a collection of all the post objects in our database by calling the all 0:08
    method on the Post class. 0:13
    We can type Post with a capital P dot all at the prompt, and 0:14
    we'll see this SQL query, SELECT posts.* FROM posts. 0:19
    That command selects all columns of all rows in the posts table. 0:24
    It's just like we saw when Post.all was called from our post controller earlier. 0:28
    Then the Rails console displays the return value of that method call, 0:33
    the collection of all post objects. 0:37
    We can get back just the most recent post object by calling the last method 0:40
    on the post class. 0:44
    If we type Post, again, with a capital P, .last at the prompt, 0:45
    we'll see the same SQL query that was generated for Post.all. 0:50
    But limited to just the most recent record. 0:54
    The ORDER BY clause sorts post by their id columns, 0:57
    the DESC sorts them in descending order, and 1:02
    the limit 1 limits the result set to just one record. 1:06
    The return value consists of just one Post model object. 1:11
    Notice that each model object has had a value assigned to its ID database column. 1:15
    We can pass that ID to the find class method 1:21
    to load a particular record from the database. 1:24
    So this post object here has an ID of 3. 1:27
    We can type Post.find with an argument of 3 and 1:30
    we'll see the SQL query SELECT posts.* 1:35
    FROM posts WHERE posts.id equals 3. 1:40
    This query is limited to just one database row, 1:44
    the one with an ID matching the ID we specified. 1:48
    The return value again consists of just one post record, the one whose ID matched. 1:51

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