ActiveModel::MissingAttributeError: missing attribute: created_at in Java

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ActiveModel::MissingAttributeError: missing attribute: created_at
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5: Working with Active Record
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565 from(*tables)
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The from method allows you to modify the table name(s) portion of the SQL statements generated by Active Record You can provide a custom value if you need to include extra tables for joins, or to reference a database view Here s an example of usage from an application that features tagging:
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def selffind_tagged_with(list) select("#{table_name}*") from("#{table_name}, tags, taggings") where("#{table_name}#{primary_key} = taggingstaggable_id and taggingstag_id = tagsid and tagsname IN ( )", Tagparse(list)]) end
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(If you re wondering why table_name is used instead of a an explicit value, it s because this code is mixed into a target class using Ruby modules That subject is covered in 9, Advanced Active Record)
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566 group(*args)
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Specifies a GROUP BY SQL-clause to add to the query generated by Active Record Generally you ll want to combine :group with the :select option, since valid SQL requires that all selected columns in a grouped SELECT be either aggregate functions or columns
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>> users = Accountselect('name, SUM(cash) as money')group('name')all => [#<User name: "Joe", money: "3500">, #<User name: "Jane", money: "9245">]
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Keep in mind that those extra columns you bring back might sometimes be strings if Active Record doesn t try to typecast them In those cases, you ll have to use to_i and to_f to explicitly convert the string to numeric types
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>> usersfirstmoney > 1_000_000 ArgumentError: comparison of String with Fixnum failed from (irb):8:in '>'
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567 having(*clauses)
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If you need to perform a group query with a SQL HAVING clause, you use the having method
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>> Usergroup("created_at")having(["created_at > ", 2daysago]) => [#<User name: "Joe", created_at: "2010-07-09 21:45:02">]
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56 Where Clauses
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568 includes(*associations)
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Active Record has the ability to eliminate N+1 queries by letting you specify what associations to eager load using the includes method or option in your finders Active Record will load those relationships with the minimum number of queries possible To eager load first degree associations, provide includes with an array of association names When accessing these a database hit to load each one will no longer occur
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>> => >> => users = Userwhere(:login => "mack")includes(:billable_weeks) [#<User login: "mack">] usersfirstbillable_weekseach { |week| puts week } #<Week start_date: "2008-05-01 00:00:00">
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For second degree associations, provide a hash with the array as the value for the hash key
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>> clients = Clientincludes(:users => [:avatar]) => [#<Client id: 1, name: "Hashrocket">]
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You may add more inclusions following the same pattern
>> Clientincludes( :users => [:avatar, { :timesheets => :billable_weeks }] ) => [#<Client id: 1, name: "Hashrocket">]
Active Record
Similarly to includes, you may use eager_load or preload with the same syntax
>> Clienteager_load( :users => [:avatar, { :timesheets => :billable_weeks }] ) => [#<Client id: 1, name: "Hashrocket">] >> Clientpreload( :users => [:avatar, { :timesheets => :billable_weeks }] ) => [#<Client id: 1, name: "Hashrocket">]
569 joins
The joins method can be useful when you re grouping and aggregating data from other tables, but you don t want to load the associated objects
Buyerselect('buyersid, count(cartsid) as cart_count') joins('left join carts on cartsbuyer_id = buyersid') group('buyersid')
However, the most common usage of the join method is to allow you to eager-fetch additional objects in a single SELECT statement, a technique that is discussed at length in 7
5: Working with Active Record
5610 readonly
Chaining the readonly method marks returned objects as read-only You can change their attributes, but you won t be able to save them back to the database
>> c = Commentreadonlyfirst => #<Comment id: 1, body: "Hey beeyotch!"> >> cbody = "Keep it clean!" => "Keep it clean!" >> csave ActiveRecord::ReadOnlyRecord: ActiveRecord::ReadOnlyRecord
5611 exists
A convenience method for checking the existence of records in the database is included in ActiveRecord as the aptly named exists method It takes similar arguments to find and instead of returning records returns a boolean for whether or not the query has results Note that the :conditions key is not used here, only supply the conditions themselves
>> Usercreate(:login => "mack") => #<User id: 1, login: "mack"> >> => >> => >> => Userexists (1) true Userexists (:login => "mack") true Userwhere(:login => "mack")exists true