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Diffstat (limited to 'guides/source/active_record_querying.md')
| -rw-r--r-- | guides/source/active_record_querying.md | 35 |
1 files changed, 31 insertions, 4 deletions
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md index cb738f0657..24dea0ec46 100644 --- a/guides/source/active_record_querying.md +++ b/guides/source/active_record_querying.md @@ -611,7 +611,8 @@ If you want to call `order` multiple times, subsequent orders will be appended t Client.order("orders_count ASC").order("created_at DESC") # SELECT * FROM clients ORDER BY orders_count ASC, created_at DESC ``` -WARNING: If you are using **MySQL 5.7.5** and above, then on selecting fields from a result set using methods like `select`, `pluck` and `ids`; the `order` method will raise an `ActiveRecord::StatementInvalid` exception unless the field(s) used in `order` clause are included in the select list. See the next section for selecting fields from the result set. + +WARNING: In most database systems, on selecting fields with `distinct` from a result set using methods like `select`, `pluck` and `ids`; the `order` method will raise an `ActiveRecord::StatementInvalid` exception unless the field(s) used in `order` clause are included in the select list. See the next section for selecting fields from the result set. Selecting Specific Fields ------------------------- @@ -807,6 +808,32 @@ SELECT * FROM articles WHERE id > 10 ORDER BY id DESC LIMIT 20 ``` +### `reselect` + +The `reselect` method overrides an existing select statement. For example: + +```ruby +Post.select(:title, :body).reselect(:created_at) +``` + +The SQL that would be executed: + +```sql +SELECT `posts`.`created_at` FROM `posts` +``` + +In case the `reselect` clause is not used, + +```ruby +Post.select(:title, :body).select(:created_at) +``` + +the SQL executed would be: + +```sql +SELECT `posts`.`title`, `posts`.`body`, `posts`.`created_at` FROM `posts` +``` + ### `reorder` The `reorder` method overrides the default scope order. For example: @@ -1526,7 +1553,7 @@ book.available? # => false ``` Read the full documentation about enums -[in the Rails API docs](http://api.rubyonrails.org/classes/ActiveRecord/Enum.html). +[in the Rails API docs](https://api.rubyonrails.org/classes/ActiveRecord/Enum.html). Understanding The Method Chaining --------------------------------- @@ -2037,9 +2064,9 @@ under MySQL and MariaDB. Interpretation of the output of EXPLAIN is beyond the scope of this guide. The following pointers may be helpful: -* SQLite3: [EXPLAIN QUERY PLAN](http://www.sqlite.org/eqp.html) +* SQLite3: [EXPLAIN QUERY PLAN](https://www.sqlite.org/eqp.html) -* MySQL: [EXPLAIN Output Format](http://dev.mysql.com/doc/refman/5.7/en/explain-output.html) +* MySQL: [EXPLAIN Output Format](https://dev.mysql.com/doc/refman/5.7/en/explain-output.html) * MariaDB: [EXPLAIN](https://mariadb.com/kb/en/mariadb/explain/) |