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author | Rolf Timmermans <r.timmermans@voormedia.com> | 2011-03-27 17:18:32 +0200 |
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committer | Rolf Timmermans <r.timmermans@voormedia.com> | 2011-03-27 17:18:32 +0200 |
commit | 512057d386075f207d8927a5e0ce3943174d5c78 (patch) | |
tree | c0b9122469af1e4af74142cc2cad6d560226753c /railties/guides/source/active_record_querying.textile | |
parent | d89a7967b5af5c87bbfc268af72287b82541d384 (diff) | |
parent | a9d27c04abf24dc85be061ff9772d71897af02b1 (diff) | |
download | rails-512057d386075f207d8927a5e0ce3943174d5c78.tar.gz rails-512057d386075f207d8927a5e0ce3943174d5c78.tar.bz2 rails-512057d386075f207d8927a5e0ce3943174d5c78.zip |
Merge remote-tracking branch 'upstream/master' into desc_tracker
Diffstat (limited to 'railties/guides/source/active_record_querying.textile')
-rw-r--r-- | railties/guides/source/active_record_querying.textile | 52 |
1 files changed, 51 insertions, 1 deletions
diff --git a/railties/guides/source/active_record_querying.textile b/railties/guides/source/active_record_querying.textile index ed3968e226..2c5d9e67e3 100644 --- a/railties/guides/source/active_record_querying.textile +++ b/railties/guides/source/active_record_querying.textile @@ -76,7 +76,7 @@ Primary operation of <tt>Model.find(options)</tt> can be summarized as: h4. Retrieving a Single Object -Active Record lets you retrieve a single object using three different ways. +Active Record lets you retrieve a single object using five different ways. h5. Using a Primary Key @@ -130,6 +130,40 @@ SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1 <tt>Model.last</tt> returns +nil+ if no matching record is found. No exception will be raised. +h5. +first!+ + +<tt>Model.first!</tt> finds the first record. For example: + +<ruby> +client = Client.first! +=> #<Client id: 1, first_name: "Lifo"> +</ruby> + +SQL equivalent of the above is: + +<sql> +SELECT * FROM clients LIMIT 1 +</sql> + +<tt>Model.first!</tt> raises +RecordNotFound+ if no matching record is found. + +h5. +last!+ + +<tt>Model.last!</tt> finds the last record. For example: + +<ruby> +client = Client.last! +=> #<Client id: 221, first_name: "Russel"> +</ruby> + +SQL equivalent of the above is: + +<sql> +SELECT * FROM clients ORDER BY clients.id DESC LIMIT 1 +</sql> + +<tt>Model.last!</tt> raises +RecordNotFound+ if no matching record is found. + h4. Retrieving Multiple Objects h5. Using Multiple Primary Keys @@ -747,6 +781,22 @@ h4. Specifying Conditions on Eager Loaded Associations Even though Active Record lets you specify conditions on the eager loaded associations just like +joins+, the recommended way is to use "joins":#joining-tables instead. +However if you must do this, you may use +where+ as you would normally. + +<ruby> +Post.includes(:comments).where("comments.visible", true) +</ruby> + +This would generate a query which contains a +LEFT OUTER JOIN+ whereas the +joins+ method would generate one using the +INNER JOIN+ function instead. + +<ruby> + SELECT "posts"."id" AS t0_r0, ... "comments"."updated_at" AS t1_r5 FROM "posts" LEFT OUTER JOIN "comments" ON "comments"."post_id" = "posts"."id" WHERE (comments.visible) +</ruby> + +If there was no +where+ condition, this would generate the normal set of two queries. + +If, in the case of this +includes+ query, there were no comments for any posts, all the posts would still be loaded. By using +joins+ (an INNER JOIN), the join conditions *must* match, otherwise no records will be returned. + h3. Scopes Scoping allows you to specify commonly-used ARel queries which can be referenced as method calls on the association objects or models. With these scopes, you can use every method previously covered such as +where+, +joins+ and +includes+. All scope methods will return an +ActiveRecord::Relation+ object which will allow for further methods (such as other scopes) to be called on it. |