diff options
Diffstat (limited to 'guides/source/active_record_querying.md')
-rw-r--r-- | guides/source/active_record_querying.md | 58 |
1 files changed, 36 insertions, 22 deletions
diff --git a/guides/source/active_record_querying.md b/guides/source/active_record_querying.md index 2f10bc4e7c..8ea0f383c0 100644 --- a/guides/source/active_record_querying.md +++ b/guides/source/active_record_querying.md @@ -10,7 +10,7 @@ After reading this guide, you will know: * How to find records using a variety of methods and conditions. * How to specify the order, retrieved attributes, grouping, and other properties of the found records. * How to use eager loading to reduce the number of database queries needed for data retrieval. -* How to use dynamic finders methods. +* How to use dynamic finder methods. * How to use method chaining to use multiple ActiveRecord methods together. * How to check for the existence of particular records. * How to perform various calculations on Active Record models. @@ -341,8 +341,6 @@ User.find_each(begin_at: 2000, batch_size: 5000) do |user| end ``` -Another example would be if you wanted multiple workers handling the same processing queue. You could have each worker handle 10000 records by setting the appropriate `:begin_at` option on each worker. - **`:end_at`** Similar to the `:begin_at` option, `:end_at` allows you to configure the last ID of the sequence whenever the highest ID is not the one you need. @@ -356,6 +354,10 @@ User.find_each(begin_at: 2000, end_at: 10000, batch_size: 5000) do |user| end ``` +Another example would be if you wanted multiple workers handling the same +processing queue. You could have each worker handle 10000 records by setting the +appropriate `:begin_at` and `:end_at` options on each worker. + #### `find_in_batches` The `find_in_batches` method is similar to `find_each`, since both retrieve batches of records. The difference is that `find_in_batches` yields _batches_ to the block as an array of models, instead of individually. The following example will yield to the supplied block an array of up to 1000 invoices at a time, with the final block containing any remaining invoices: @@ -390,7 +392,7 @@ Now what if that number could vary, say as an argument from somewhere? The find Client.where("orders_count = ?", params[:orders]) ``` -Active Record will go through the first element in the conditions value and any additional elements will replace the question marks `(?)` in the first element. +Active Record will take the first argument as the conditions string and any additional arguments will replace the question marks `(?)` in it. If you want to specify multiple conditions: @@ -418,7 +420,7 @@ TIP: For more information on the dangers of SQL injection, see the [Ruby on Rail #### Placeholder Conditions -Similar to the `(?)` replacement style of params, you can also specify keys/values hash in your array conditions: +Similar to the `(?)` replacement style of params, you can also specify keys in your conditions string along with a corresponding keys/values hash: ```ruby Client.where("created_at >= :start_date AND created_at <= :end_date", @@ -429,7 +431,7 @@ This makes for clearer readability if you have a large number of variable condit ### Hash Conditions -Active Record also allows you to pass in hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want conditionalised and the values of how you want to conditionalise them: +Active Record also allows you to pass in hash conditions which can increase the readability of your conditions syntax. With hash conditions, you pass in a hash with keys of the fields you want qualified and the values of how you want to qualify them: NOTE: Only equality, range and subset checking are possible with Hash conditions. @@ -529,7 +531,7 @@ Client.order("orders_count ASC, created_at DESC") Client.order("orders_count ASC", "created_at DESC") ``` -If you want to call `order` multiple times e.g. in different context, new order will append previous one: +If you want to call `order` multiple times, subsequent orders will be appended to the first: ```ruby Client.order("orders_count ASC").order("created_at DESC") @@ -617,9 +619,9 @@ SELECT * FROM clients LIMIT 5 OFFSET 30 Group ----- -To apply a `GROUP BY` clause to the SQL fired by the finder, you can specify the `group` method on the find. +To apply a `GROUP BY` clause to the SQL fired by the finder, you can use the `group` method. -For example, if you want to find a collection of the dates orders were created on: +For example, if you want to find a collection of the dates on which orders were created: ```ruby Order.select("date(created_at) as ordered_date, sum(price) as total_price").group("date(created_at)") @@ -637,7 +639,7 @@ GROUP BY date(created_at) ### Total of grouped items -To get the total of grouped items on a single query call `count` after the `group`. +To get the total of grouped items on a single query, call `count` after the `group`. ```ruby Order.group(:status).count @@ -673,7 +675,7 @@ GROUP BY date(created_at) HAVING sum(price) > 100 ``` -This will return single order objects for each day, but only those that are ordered more than $100 in a day. +This returns the date and total price for each order object, grouped by the day they were ordered and where the price is more than $100. Overriding Conditions --------------------- @@ -703,8 +705,7 @@ Article.where(id: 10, trashed: false).unscope(where: :id) # SELECT "articles".* FROM "articles" WHERE trashed = 0 ``` -A relation which has used `unscope` will affect any relation it is -merged in to: +A relation which has used `unscope` will affect any relation into which it is merged: ```ruby Article.order('id asc').merge(Article.unscope(:order)) @@ -748,7 +749,7 @@ SELECT * FROM articles WHERE id = 10 SELECT * FROM comments WHERE article_id = 10 ORDER BY name ``` -In case the `reorder` clause is not used, the SQL executed would be: +In the case where the `reorder` clause is not used, the SQL executed would be: ```sql SELECT * FROM articles WHERE id = 10 @@ -837,7 +838,7 @@ end Readonly Objects ---------------- -Active Record provides `readonly` method on a relation to explicitly disallow modification of any of the returned objects. Any attempt to alter a readonly record will not succeed, raising an `ActiveRecord::ReadOnlyRecord` exception. +Active Record provides the `readonly` method on a relation to explicitly disallow modification of any of the returned objects. Any attempt to alter a readonly record will not succeed, raising an `ActiveRecord::ReadOnlyRecord` exception. ```ruby client = Client.readonly.first @@ -898,7 +899,7 @@ For example: Item.transaction do i = Item.lock.first i.name = 'Jones' - i.save + i.save! end ``` @@ -1050,7 +1051,7 @@ SELECT categories.* FROM categories ### 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) provides a special syntax for specifying conditions for 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: ```ruby time_range = (Time.now.midnight - 1.day)..Time.now.midnight @@ -1089,7 +1090,7 @@ This code looks fine at the first sight. But the problem lies within the total n Active Record lets you specify in advance all the associations that are going to be loaded. This is possible by specifying the `includes` method of the `Model.find` call. With `includes`, Active Record ensures that all of the specified associations are loaded using the minimum possible number of queries. -Revisiting the above case, we could rewrite `Client.limit(10)` to use eager load addresses: +Revisiting the above case, we could rewrite `Client.limit(10)` to eager load addresses: ```ruby clients = Client.includes(:address).limit(10) @@ -1266,6 +1267,18 @@ class Client < ActiveRecord::Base end ``` +NOTE: The `default_scope` is also applied while creating/building a record. +It is not applied while updating a record. E.g.: + +```ruby +class Client < ActiveRecord::Base + default_scope { where(active: true) } +end + +Client.new # => #<Client id: nil, active: true> +Client.unscoped.new # => #<Client id: nil, active: nil> +``` + ### Merging of scopes Just like `where` clauses scopes are merged using `AND` conditions. @@ -1414,7 +1427,7 @@ It's common that you need to find a record or create it if it doesn't exist. You ### `find_or_create_by` -The `find_or_create_by` method checks whether a record with the attributes exists. If it doesn't, then `create` is called. Let's see an example. +The `find_or_create_by` method checks whether a record with the specified attributes exists. If it doesn't, then `create` is called. Let's see an example. Suppose you want to find a client named 'Andy', and if there's none, create one. You can do so by running: @@ -1787,8 +1800,9 @@ EXPLAIN for: SELECT `users`.* FROM `users` INNER JOIN `articles` ON `articles`.` under MySQL. -Active Record performs a pretty printing that emulates the one of the database -shells. So, the same query running with the PostgreSQL adapter would yield instead +Active Record performs a pretty printing that emulates that of the +corresponding database shell. So, the same query running with the +PostgreSQL adapter would yield instead ``` EXPLAIN for: SELECT "users".* FROM "users" INNER JOIN "articles" ON "articles"."user_id" = "users"."id" WHERE "users"."id" = 1 @@ -1853,6 +1867,6 @@ following pointers may be helpful: * SQLite3: [EXPLAIN QUERY PLAN](http://www.sqlite.org/eqp.html) -* MySQL: [EXPLAIN Output Format](http://dev.mysql.com/doc/refman/5.6/en/explain-output.html) +* MySQL: [EXPLAIN Output Format](http://dev.mysql.com/doc/refman/5.7/en/explain-output.html) * PostgreSQL: [Using EXPLAIN](http://www.postgresql.org/docs/current/static/using-explain.html) |