>> => >> => 01+02 == 03 false BigDecimal('01') + BigDecimal('02') == BigDecimal('03') true in Java

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>> => >> => 01+02 == 03 false BigDecimal('01') + BigDecimal('02') == BigDecimal('03') true
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Active Record
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:float Don t use floats to store currency values, or more accurately, any type of data
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that needs fixed precision Since floating-point numbers are pretty much approximations, any single representation of a number as a float is probably okay However, once you start doing mathematical operations or comparisons with float values, it is ridiculously easy to introduce difficult to diagnose bugs into your application
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:integer and :string There aren t many gotchas that I can think of when it comes
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to integers and strings They are the basic data building blocks of your application, and many Rails developers leave off the size specification, which results in the default maximum sizes of 11 digits and 255 characters, respectively You should keep in mind that you won t get an error if you try to store values that exceed the maximum size defined for the database column, which again, is 255 characters by default Your string will simply get truncated Use validations to make sure that user-entered data does not exceed the maximum size allowed
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:text There have been reports of text fields slowing down query performance on some
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databases, enough to be a consideration for applications that need to scale to high loads If you must use a text column in a performance-critical application, put it in a separate table
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Custom Data Types If use of database-specific datatypes (such as :double, for higher precision than :float) is critical to your project, use the configactive_recordschema_format = :sql
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6: Active Record Migrations
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setting in config/applicationrb to make Rails dump schema information in native SQL DDL format rather than its own cross-platform compatible Ruby code, via the db/schemarb file
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Magic Timestamp Columns Rails does magic with datetime columns, if they re named a certain way Active Record will automatically timestamp create operations if the table has columns named created_at or created_on The same applies to updates when there are columns named updated_at or updated_on Note that created_at and updated_at should be defined as datetime, but if you use ttimestamps then you don t have to worry about what type of columns they are Automatic timestamping can be turned off globally, by setting the following variable in an initializer
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ActiveRecord::Baserecord_timestamps = false
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The preceding code turns off timestamps for all models, but record_timestamps is class-inheritable, so you can also do it on a case-by-case basis by setting selfrecord_timestamps to false at the top of specific model classes
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617 Command-line Column Declarations
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You can supply name/type pairs on the command line when you invoke the migration generator and it will automatically insert the corresponding add_column and remove_column methods
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$ rails generate migration AddTitleBodyToPosts \ title:string body:text published:boolean
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This will create the AddTitleBodyToPosts in db/migrate/20080514090912_ add_title_body_to_postsrb with this in the up migration:
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add_column :posts, :title, :string add_column :posts, :body, :text add_column :posts, :published, :boolean
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And this in the down migration:
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remove_column :posts, :published remove_column :posts, :body remove_column :posts, :title
62 Data Migration
62 Data Migration
So far we ve only discussed using migration files to modify the schema of your database Inevitably, you will run into situations where you also need to perform data migrations, whether in conjunction with a schema change or not
621 Using SQL
In most cases, you should craft your data migration in raw SQL using the execute command that is available inside a migration class For example, say you had a phones table, which kept phone numbers in their component parts and later wanted to simplify your model by just having a number column instead You d write a migration similar to this one (only the up method is shown, for brevity):
class CombineNumberInPhones < ActiveRecord::Migration def selfup add_column :phones, :number, :string execute("update phones set number = concat(area_code, prefix, suffix)") remove_column :phones, :area_code remove_column :phones, :prefix remove_column :phones, :suffix end end
Active Record
The naive alternative to using SQL in the example above would be more lines of code and much, much slower
Phonefind_each do |p| pnumber = parea_code + pprefix + psuffix psave end
In this particular case, you could use Active Record s update_all method to still do the data migration in one line
Phoneupdate_all("set number = concat(area_code, prefix, suffix)")
However you might hit problems down the road as your schema evolves; as described in the next section, you d want to declare an independent Phone model in the migration file itself That s why I advise sticking to raw SQL whenever possible