Ruby on Rails Where to define global constants

Question

I m just getting started with my first Ruby on Rails webapp  I ve got a bunch of different models  views  controllers  and so on    I m wanting to find a good place to stick definitions of truly global constants  that apply across my whole app  In particular  they apply both in the logic of my models  and in the decisions taken in my views  I cannot find any DRY place to put these definitions where they re available both to all my models and also in all my views   To take a specific example  I want a constant COLOURS     white    blue    black    red    green    This is used all over the place  in both models and views  Where can I define it in just one place so that it s accessible   What I ve tried    Constant class variables in the model rb file that they re most associated with  such as   COLOURS          But I couldn t find a sane way to define it so that I can write in my views Card COLOURS rather than something kludgy like Card first COLOURS  A method on the model  something like def colours   white       end - same problem  A method in application helper rb - this is what I m doing so far  but the helpers are only accessible in views  not in models I think I might have tried something in application rb or environment rb  but those don t really seem right  and they don t seem to work either    Is there just no way to define anything to be accessible both from models and from views  I mean  I know models and views should be separate  but surely in some domains there ll be times they need to refer to the same domain-specific knowledge

User · Answer

The global variable should be declare in config initializers directory  COLOURS    w white blue black red green

User · Answer

According your condition  you can also define some environmental variables  and fetch it via ENV  some-var   in ruby code   this solution may not fit for you  but I hope it may help others       Example  you can create different files  development env   production env   test env and load it according your application environments  check this gen dotenv-rails which automate this for your

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For application-wide settings and for global constants I recommend to use Settingslogic  This settings are stored in YML file and can be accessed from models  views and controllers  Furthermore  you can create different settings for all your environments       app config application yml   defaults   amp defaults     cool        sweet  nested settings     neat setting  24     awesome setting   lt     Did you know 5   5     5   5      gt       colors   white blue black red green     development       lt  lt    defaults     neat setting  800    test       lt  lt    defaults    production       lt  lt    defaults   Somewhere in the view  I prefer helper methods for such kind of stuff  or in a model you can get  for ex   array of colors Settings colors split   s    It s very flexible  And you don t need to invent a bike

User · Answer

Some options   Using a constant   class Card   COLOURS     white    blue    black    red    green    yellow   freeze end   Lazy loaded using class instance variable   class Card   def self colours      colours       white    blue    black    red    green    yellow   freeze   end end   If it is a truly global constant  avoid global constants of this nature  though   you could also consider putting a top-level constant in config initializers my constants rb for example

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Another option  if you want to define your constants in one place   module DSL   module Constants     MY CONSTANT   1   end end   But still make them globally visible without having to access them in fully qualified way   DSL  Constants  MY CONSTANT     gt  1 MY CONSTANT     gt  NameError  uninitialized constant MY CONSTANT Object instance eval   include DSL  Constants   MY CONSTANT     gt  1

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If your model is really  quot responsible quot  for the constants you should stick them there  You can create class methods to access them without creating a new object instance  class Card  lt  ActiveRecord  Base   def self colours       white    blue     end end    accessible like this Card colours  Alternatively  you can create class variables and an accessor  This is however discouraged as class variables might act kind of surprising with inheritance and in multi-thread environments  class Card  lt  ActiveRecord  Base     colours     white    blue   freeze   cattr reader  colours end    accessible the same as above Card colours  The two options above allow you to change the returned array on each invocation of the accessor method if required  If you have true a truly unchangeable constant  you can also define it on the model class  class Card  lt  ActiveRecord  Base   COLOURS     white    blue   freeze end    accessible as Card  COLOURS  You could also create global constants which are accessible from everywhere in an initializer like in the following example  This is probably the best place  if your colours are really global and used in more than one model context    put this into config initializers my constants rb COLOURS     white    blue   freeze    accessible as a top-level constant this time COLOURS  Note  when we define constants above  often we want to freeze the array  That prevents other code from later  inadvertently  modifying the array by e g  adding a new element  Once an object is frozen  it can t be changed anymore

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As of Rails 4 2  you can use the config x property     config application rb  or config custom rb if you prefer  config x colours options    w white blue black red green  config x colours default    white    Which will be available as   Rails configuration x colours options     gt    white    blue    black    red    green   Rails configuration x colours default     gt   white    Another method of loading custom config     config colours yml default   amp default   options      - white     - blue     - black     - red     - green   default  white development     default production     default     config application rb config colours   config for  colours    Rails configuration colours     gt    options   gt   white    blue    black    red    green     default   gt  white   Rails configuration colours  default       gt   white      In Rails 5  amp  6  you can use the configuration object directly for custom configuration  in addition to config x  However  it can only be used for non-nested configuration     config application rb config colours    w white blue black red green    It will be available as   Rails configuration colours     gt    white    blue    black    red    green

User · Answer

Use a class method   def self colours     white    red    black   end   Then Model colours will return that array  Alternatively  create an initializer and wrap the constants in a module to avoid namespace conflicts

User · Answer

Try to keep all constant at one place  In my application I have created constants folder inside initializers as follows     and I usually keep all constant in these files   In your case you can create file under constants folder as colors constant rb  colors constant rb    Don t forgot to restart server

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If a constant is needed in more than one class  I put it in config initializers contant rb always in all caps  list of states below is truncated    STATES     AK    AL        WI    WV    WY     They are available through out the application except in model code as such        lt    form label  states    gt       lt    form select  states  STATES       gt    To use the constant in a model  use attr accessor to make the constant available   class Customer  lt  ActiveRecord  Base     attr accessor  STATES      validates  state  inclusion   in  STATES  message   -- choose a State from the drop down list    end

User · Answer

I typically have a  lookup  model table in my rails program and use it for the constants  It is very useful if the constants are going to be different for different environments  In addition  if you have a plan to extend them  say you want to add  yellow  on a later date  you could simply add a new row to the lookup table and be done with it   If you give the admin permissions to modify this table  they will not come to you for maintenance     DRY   Here is how my migration code looks like   class CreateLookups  lt  ActiveRecord  Migration   def change     create table  lookups do  t        t string  group key       t string  lookup key       t string  lookup value       t timestamps     end   end end   I use seeds rb to pre-populate it   Lookup find or create by group key and lookup key and lookup value  group key   development COLORS   lookup key   color1   lookup value   red

User · Answer

A common place to put application-wide global constants is inside config application   module MyApp   FOO     ENV fetch  FOO   nil    BAR      w one two three     class Application  lt  Rails  Application     config foo bar    baz   end end

[ruby-on-rails] Ruby on Rails: Where to define global constants?

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