Reading and Writing Attributes in Java

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543 Reading and Writing Attributes
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After you have retrieved a model instance from the database, you can access each of its columns in several ways The easiest (and clearest to read) is simply with dot notation:
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>> => >> => first_clientname "Paper Jam Printers" first_clientcode "PJP"
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54 CRUD: Creating, Reading, Updating, Deleting
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The private read_attribute method of Active Record, covered briefly in an earlier section, is useful to know about, and comes in handy when you want to override a default attribute accessor To illustrate, while still in the Rails console, I ll go ahead and reopen the Client class on the fly and override the name accessor to return the value from the database, but reversed:
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>> >> >> >> >> => >> => class Client < ActiveRecord::Base def name read_attribute(:name)reverse end end nil first_clientname "sretnirP maJ repaP"
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Hopefully it s not too painfully obvious for me to demonstrate why you need read_attribute in that scenario Recursion is a bitch, if it s unexpected:
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Active Record
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>> class Client < ActiveRecord::Base >> def name >> selfnamereverse >> end >> end => nil >> first_clientname SystemStackError: stack level too deep from (irb):21:in 'name' >> class Client < ActiveRecord::Base from (irb):21:in 'name' >> class Client < ActiveRecord::Base from (irb):24
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As can be expected by the existence of a read_attribute method (and as we covered earlier in the chapter), there is also a write_attribute method that lets you change attribute values Just as with attribute getter methods, you can override the setter methods and provide your own behavior:
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class Project < ActiveRecord::Base # The description for a project cannot be changed to a blank string def description=(new_value) write_attribute(:description, new_value) unless new_valueblank end end
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The preceding example illustrates a way to do basic validation, since it checks to make sure that a value is not blank before allowing assignment However, as we ll see in 8, Validations, there are better ways to do this
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5: Working with Active Record
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Hash Notation Yet another way to access attributes is using the [attribute_name] operator, which lets you access the attribute as if it were a regular hash
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>> => >> => first_client['name'] "Paper Jam Printers" first_client[:name] "Paper Jam Printers"
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Many Rails methods accept symbol and string parameters interchangeably, and that is potentially very confusing Which is more correct The general rule is to use symbols when the string is a name for something, and a string when it s a value You should probably be using symbols when it comes to keys of options hashes and the like
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The attributes Method There is also an attributes method that returns a hash with each attribute and its corresponding value as returned by read_attribute If you use your own custom attribute reader and writer methods, it s important to remember that attributes will not use custom attribute readers when accessing its values, but attributes= (which lets you do mass assignment) does invoke custom attribute writers
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>> first_clientattributes => {"name"=>"Paper Jam Printers", "code"=>"PJP", "id"=>1}
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Being able to grab a hash of all attributes at once is useful when you want to iterate over all of them or pass them in bulk to another function Note that the hash returned from attributes is not a reference to an internal structure of the Active Record object It is copy, which means that changing its values will have no effect on the object it came from
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>> => >> => >> => atts = first_clientattributes {"name"=>"Paper Jam Printers", "code"=>"PJP", "id"=>1} atts["name"] = "Def Jam Printers" "Def Jam Printers" first_clientattributes {"name"=>"Paper Jam Printers", "code"=>"PJP", "id"=>1}
To make changes to an Active Record object s attributes in bulk, it is possible to pass a hash to the attributes writer
54 CRUD: Creating, Reading, Updating, Deleting
544 Accessing and Manipulating Attributes Before They Are Typecast
The Active Record connection adapters, classes that implement behavior specific to databases, fetch results as strings Rails then takes care of converting them to other datatypes if necessary, based on the type of the database column For instance, integer types are cast to instances of Ruby s Fixnum class, and so on Even if you re working with a new instance of an Active Record object, and have passed in constructor values as strings, they will be typecast to their proper type when you try to access those values as attributes Sometimes you want to be able to read (or manipulate) the raw attribute data without having the column-determined typecast run its course first, and that can be done by using the attribute_before_type_cast accessors that are automatically created in your model For example, consider the need to deal with currency strings typed in by your end users Unless you are encapsulating currency values in a currency class (highly recommended, by the way) you need to deal with those pesky dollar signs and commas Assuming that our Timesheet model had a rate attribute defined as a :decimal type, the following code would strip out the extraneous characters before typecasting for the save operation:
class Timesheet < ActiveRecord::Base before_validation :fix_rate def fix_rate self[:rate] = rate_before_type_casttr('$,','') end end