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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
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out of order in our tour of CRUD operations and start with read operations.
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We can get a collection of all the post objects in our database by calling the all
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method on the Post class.
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We can type Post with a capital P dot all at the prompt, and
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we'll see this SQL query, SELECT posts.* FROM posts.
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That command selects all columns of all rows in the posts table.
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It's just like we saw when post.all was called from our post controller earlier.
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Then the Rails console displays the return value of that method call,
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the collection of all post objects.
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We can get back just the most recent post object by calling the last method
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on the post class.
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If we type post, again, with a capital P, .last at the prompt,
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we'll see the same SQL query that was generated for post.all.
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But limited to just the most recent record.
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The ORDER BY clause sorts post by their id columns,
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the DESC sorts them in descending order, and
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the limit 1 limits the result set to just one record.
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The return value consists of just one Post model object.
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Notice that each model object has had a value assigned to its ID database column.
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We can pass that ID to the find class method
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to load a particular record from the database.
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So this post object here has an ID of 3.
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We can type Post.find with an argument of 3 and
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we'll see the SQL query SELECT posts.*
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FROM posts WHERE posts.id equals 3.
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This query is limited to just one database row,
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the one with an ID matching the ID we specified.
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The return value again consists of just one post record, the one whose ID matched.
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