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Diffstat (limited to 'guides/source')
-rw-r--r-- | guides/source/active_record_querying.md | 48 |
1 files changed, 37 insertions, 11 deletions
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md index 1427903dfb..b7773ea65a 100644 --- a/guides/source/active_record_querying.md +++ b/guides/source/active_record_querying.md @@ -69,6 +69,7 @@ The methods are: * `having` * `includes` * `joins` +* `left_outer_joins` * `limit` * `lock` * `none` @@ -935,25 +936,30 @@ end Joining Tables -------------- -Active Record provides a finder method called `joins` for specifying `JOIN` clauses on the resulting SQL. There are multiple ways to use the `joins` method. +Active Record provides two finder methods for specifying `JOIN` clauses on the +resulting SQL: `joins` and `left_outer_joins`. +While `joins` should be used for `INNER JOIN` or custom queries, +`left_outer_joins` is used for queries using `LEFT OUTER JOIN`. -### Using a String SQL Fragment +### `joins` + +There are multiple ways to use the `joins` method. + +#### Using a String SQL Fragment You can just supply the raw SQL specifying the `JOIN` clause to `joins`: ```ruby -Client.joins('LEFT OUTER JOIN addresses ON addresses.client_id = clients.id') +Author.joins("INNER JOIN posts ON posts.author_id = author.id AND posts.published = 't'") ``` This will result in the following SQL: ```sql -SELECT clients.* FROM clients LEFT OUTER JOIN addresses ON addresses.client_id = clients.id +SELECT clients.* FROM clients INNER JOIN posts ON posts.author_id = author.id AND posts.published = 't' ``` -### Using Array/Hash of Named Associations - -WARNING: This method only works with `INNER JOIN`. +#### Using Array/Hash of Named Associations Active Record lets you use the names of the [associations](association_basics.html) defined on the model as a shortcut for specifying `JOIN` clauses for those associations when using the `joins` method. @@ -986,7 +992,7 @@ end Now all of the following will produce the expected join queries using `INNER JOIN`: -#### Joining a Single Association +##### Joining a Single Association ```ruby Category.joins(:articles) @@ -1017,7 +1023,7 @@ SELECT articles.* FROM articles Or, in English: "return all articles that have a category and at least one comment". Note again that articles with multiple comments will show up multiple times. -#### Joining Nested Associations (Single Level) +##### Joining Nested Associations (Single Level) ```ruby Article.joins(comments: :guest) @@ -1033,7 +1039,7 @@ SELECT articles.* FROM articles Or, in English: "return all articles that have a comment made by a guest." -#### Joining Nested Associations (Multiple Level) +##### Joining Nested Associations (Multiple Level) ```ruby Category.joins(articles: [{ comments: :guest }, :tags]) @@ -1049,7 +1055,7 @@ SELECT categories.* FROM categories INNER JOIN tags ON tags.article_id = articles.id ``` -### Specifying Conditions on the Joined Tables +#### Specifying Conditions on the Joined Tables You can specify conditions on the joined tables using the regular [Array](#array-conditions) and [String](#pure-string-conditions) conditions. [Hash conditions](#hash-conditions) provide a special syntax for specifying conditions for the joined tables: @@ -1067,6 +1073,26 @@ Client.joins(:orders).where(orders: { created_at: time_range }) This will find all clients who have orders that were created yesterday, again using a `BETWEEN` SQL expression. +### `left_outer_joins` + +If you want to select a set of records whether or not they have associated +records you can use the `left_outer_joins` method. + +```ruby +Author.left_outer_joins(:posts).uniq.select('authors.*, COUNT(posts.*) AS posts_count').group('authors.id') +``` + +Which produces: + +```sql +SELECT DISTINCT authors.*, COUNT(posts.*) AS posts_count FROM "authors" +LEFT OUTER JOIN posts ON posts.author_id = authors.id GROUP BY authors.id +``` + +Which means: "return all authors with their count of posts, whether or not they +have any posts at all" + + Eager Loading Associations -------------------------- |