Ruby on Rails - Array of Hashes - Display value based on key

The problem appears to be that the values for the keys :financial_year in your array of hashes are strings (e.g. "2017"), but your value for the variable financial_year is a fixnum/integer (e.g 2017). Try making them consistent to compare, such as:

<%= @item.yearly_interest_totals.detect do |t|
  t[:financial_year] == financial_year.to_s end [:total] %>

Here is output from the Rails console comparing the two:

Running via Spring preloader in process 15647
Loading development environment (Rails 4.2.7.1)

2.3.3 :001 > item_yearly_interest_totals = [{ financial_year: "2017", total: "120.08" }, { financial_year: "2018", total: "237.32" }, { financial_year: "2019", total: "163.75" }, { financial_year: "2020", total: "87.95" }, { financial_year: "2021", total: "15.38" }]
=> [{:financial_year=>"2017", :total=>"120.08"}, {:financial_year=>"2018", :total=>"237.32"}, {:financial_year=>"2019", :total=>"163.75"}, {:financial_year=>"2020", :total=>"87.95"}, {:financial_year=>"2021", :total=>"15.38"}]

2.3.3 :002 > financial_year = 2017
=> 2017

2.3.3 :003 > item_yearly_interest_totals.detect do |t|
2.3.3 :004 >      t[:financial_year] == financial_year end [:total]
NoMethodError: undefined method `[]' for nil:NilClass
  .
  .
  .

2.3.3 :005 > item_yearly_interest_totals.detect do |t|
2.3.3 :006 >      t[:financial_year] == financial_year.to_s end [:total]
=> "120.08"

2.3.3 :007 > 

UPDATE (02-20-2017)

I don't completely understand where the distinction within Rails lies or is occurring that is the source of your issue, but even though you execute @item.yearly_interest_totals[0].class and you get Hash, you can't seem to access the values using a hash key (e.g. [:financial_year], ["financial_year"], etc.).

After some digging, I found this: Rails access hash value and the accepted answer led me to try JSON.parse, which I was able to get working, albeit with .each rather than .detect. This time I did, in a Rails 5 app, create an Item model, used Postgres, and seeded a single Item. What I still did not do is create a controller or any views. I executed my code through the Rails console. So, if you duplicate my code and it does not work for you, the problem may lie there, within the controller and views.

Ultimately, there is still some discovery to be done regarding this hash/JSON distinction and how implementation leads it to manifest as one or the other.

app/models/item.rb

class Item < ApplicationRecord
  validates :name, presence: true
end

db/migrate/20170220221004_enable_hstore_extension.rb

class EnableHstoreExtension < ActiveRecord::Migration
  def change
    enable_extension 'hstore'
  end
end

db/migrate/20170220221129_create_item.rb

class CreateItem < ActiveRecord::Migration[5.0]
  def change
    create_table :items do |t|
      t.string :name, null: false, index: { unique: true }
      t.hstore :yearly_interest_totals, array: true
      t.timestamps null: false
    end
  end
end

db/seeds.rb

Item.create(name: 'Sample Item', yearly_interest_totals: [{ financial_year: "2017", total: "120.08" }, { financial_year: "2018", total: "237.32" }, { financial_year: "2019", total: "163.75" }, { financial_year: "2020", total: "87.95" }, { financial_year: "2021", total: "15.38" }])

And here is the code as it is executed in the Rails console:

Running via Spring preloader in process 19764
Loading development environment (Rails 5.0.1)

2.4.0 :001 > @item = Item.first
Item Load (1.4ms)  SELECT  "items".* FROM "items" ORDER BY "items"."id" ASC LIMIT $1  [["LIMIT", 1]]
=> #<Item id: 1, name: "Sample Item", yearly_interest_totals: [{"total"=>"120.08", "financial_year"=>"2017"}, {"total"=>"237.32", "financial_year"=>"2018"}, {"total"=>"163.75", "financial_year"=>"2019"}, {"total"=>"87.95", "financial_year"=>"2020"}, {"total"=>"15.38", "financial_year"=>"2021"}], created_at: "2017-02-20 22:25:14", updated_at: "2017-02-20 22:25:14">
2.4.0 :002 > @item.class
=> Item(id: integer, name: string, yearly_interest_totals: hstore, created_at: datetime, updated_at: datetime)
2.4.0 :003 > @item.yearly_interest_totals.class
=> Array
2.4.0 :004 > @item.yearly_interest_totals[0].class
=> Hash

2.4.0 :005 > financial_year = 2017
=> 2017
2.4.0 :006 > financial_year.class
=> Integer

2.4.0 :007 > selected_year_interest_total = nil
=> nil
2.4.0 :008 > selected_year_interest_total.class
=> NilClass

2.4.0 :009 > @item.yearly_interest_totals.each do |t|
2.4.0 :010 >   puts JSON.parse(t["financial_year"]).class
2.4.0 :011 >   if JSON.parse(t["financial_year"]) == financial_year
2.4.0 :012?>     selected_year_interest_total = JSON.parse(t["total"])
2.4.0 :013?>   end
2.4.0 :014?> end
Integer
Integer
Integer
Integer
Integer
=> [{"total"=>"120.08", "financial_year"=>"2017"}, {"total"=>"237.32", "financial_year"=>"2018"}, {"total"=>"163.75", "financial_year"=>"2019"}, {"total"=>"87.95", "financial_year"=>"2020"}, {"total"=>"15.38", "financial_year"=>"2021"}]

2.4.0 :015 > selected_year_interest_total
=> 120.08
2.4.0 :016 > selected_year_interest_total.class
=> Float