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Web applications scale best with optimistic locking, which as we ve discussed doesn t really use any database-level locking at all However, you have to add application logic to handle failure cases Pessimistic locking is a bit easier to implement, but can lead to situations where one Rails process is waiting on another to release a database lock, that is, waiting and not serving any other incoming requests Remember that Rails processes are typically single-threaded In my opinion, pessimistic locking should not be super dangerous as it is on other platforms, because in Rails we don t ever persist database transactions across more than a single HTTP request In fact, it would be impossible to do that in a shared-nothing architecture (If you re running Rails with JRuby and doing crazy things like storing Active Record object instances in a shared session space, all bets are off)
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5: Working with Active Record
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A situation to be wary of would be one where you have many users competing for access to a particular record that takes a long time to update For best results, keep your pessimistic-locking transactions small and make sure that they execute quickly
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56 Where Clauses
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In mentioning Active Record s find method earlier in the chapter, we didn t look at the wealth of options available in addition to finding by primary key and the first and all methods Note that this book covers a querying style that is new to Rails 3 Each method discussed here returns an ActiveRecord::Relation - a chainable object that is lazy evaluated against the database only when the actual records are needed
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561 where(*conditions)
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It s very common to need to filter the result set of a find operation (just a SQL SELECT under the covers) by adding conditions (to the WHERE clause) Active Record gives you a number of ways to do just that with the where method The conditions parameter can be specified as a string or a hash Parameters are automatically santized to prevent SQL-injection attacks Passing a hash of conditions will construct a where clause containing a union of all the key/value pairs If all you need is equality, versus, say LIKE criteria, I advise you to use the hash notation, since it s arguably the most readable of the styles
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Productwhere(:sku => params[:sku))
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The hash notation is smart enough to create an IN clause if you associate an array of values with a particular key
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Productwhere(:sku => [9400,9500,9900])
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The simple string form can be used for statements that don t involve data originating outside of your app It s most useful for doing LIKE comparsions, as well as greaterthan/less-than and the use of SQL functions not already built into Active Record If you do choose to use the string style, additional arguments to the where method will be treated as query variables to insert into the where clause
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Productwhere('description like and color = ', "%#{terms}%", color) Productwhere('sku in ( )', selected_skus)
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Bind Variables When using multiple parameters in the conditions, it can easily become hard to read exactly what the fourth or fifth question mark is supposed to represent In those cases, you can resort to named bind variables instead That s done by replacing the question marks with symbols and supplying a hash with values for the matching symbol keys as a second parameter
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Productwhere("name = :name AND sku = :sku AND created_at > :date", :name => "Space Toilet", :sku => 80800, :date => '2009-01-01')
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During a quick discussion on IRC about this final form, Robby Russell gave me the following clever snippet:
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Messagewhere("subject LIKE :foo OR body LIKE :foo", :foo => '%woah%')
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In other words, when you re using named placeholders (versus question mark characters) you can use the same bind variable more than once Like, whoa! Simple hash conditions like this are very common and useful, but they will only generate conditions based on equality with SQL s AND operator
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