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def selfup # Phone number fields are not integers, duh! change_column :clients, :phone, :string end def selfdown raise ActiveRecord::IrreversibleMigration end

Getting back to the Migration API itself, here is the 20090124223305_create_ clientsrb file again, from earlier in the chapter, after adding a couple of column definitions for the clients table:

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class CreateClients < ActiveRecord::Migration def selfup create_table :clients do |t| tstring :name tstring :code ttimestamps end end def selfdown drop_table :clients end end

As you can see in the example, migration directives happen within two class method definitions, selfup and selfdown If we go to the command line in our project folder and type rake db:migrate, the clients table will be created Rails gives us informative output during the migration process so that we see what is going on:

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and other explicitly unsigned declarations DWORD and its peers are also suitable unsigned types on Microsoft compilers Using unsigned variables means that you only need to check against an upper bound to verify that a value is within a reasonable range3 A variety of implicit conversion rules combat any effort to ensure that a speci c number is always positive We sympathize with the frustration Bjarne Stroustrup, the designer and original implementer of C++, expressed when he said that attempts to ensure that some values are positive by declaring variables unsigned will typically be defeated by the implicit conversion rules [Stroustrup, 2000] However, unsigned types are too valuable in preventing integer over ow to leave by the wayside To combat errors caused by undesirable implicit conversions, be consistent with the sign of every value used in comparisons and arithmetic operations, and remember to use unsigned constants to ensure that these operations behave as intended Expect Bad Assumptions Keep in mind that standard types such as int, char, and size_t have different de nitions depending on the platform and compiler being used Declarations that do not explicitly specify a sign rely on the compiler default, which, in the case of char, could be signed in some cases and unsigned in others The size of these standard types can also vary from one system to another, which can cause unanticipated sign extension problems, particularly when code is compiled on both 32-bit and 64-bit platforms Often the solution to both problems is to make code resilient to changes in the underlying representation of an integral type by declaring its signedness explicitly, which can prevent the unexpected sign extension shown in Example 73 Restrict Numeric User Input Restrict the range of numeric input that you accept Just because a 32-bit unsigned integer can be used to represent the number four billion doesn t mean that users should be allowed to claim they have four billion fonts in one document or four billion items in one shopping cart Impose reasonable maximums and minimums.

<composite name="CreditComposite" xmlns:bb="http://bigbankcom"> <component name=" CreditServiceComponent"> <implementationcomposite name=" loan:CreditServiceComposite"/> <property name="validationRules" le="validationRulesxml"/> </component> </composite>

$ rake db:migrate == CreateClients: migrating ========================================== -- create_table(:clients) -> 00448s == CreateClients: migrated (00450s) =================================

Normally, only the code in the up method is run, but if you ever need to rollback to an earlier version of the schema, the down method specifies how to undo what happened in up To execute a rollback, use the migrate task, but pass it a version number to rollback to, as in rake db:migrate VERSION=20090124223305

3 The lack of unsigned types is a drawback of Java See the subsection on sanity-checking arithmetic operations for evidence of the additional complexity involved in validating signed operations

613 create table(name, options, & block)

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