Seed file

gistfile1.rb

category_attributes = [{name: 'Wrecks/Artifacts', image: Rails.root.join('/assets/images/categories/Wrecks.jpg')},
 {name: 'Caves', image: Rails.root.join('/app/assets/images/categories/Caves.jpg')},
 {name: 'Drift', image: Rails.root.join('/app/assets/images/categories/Drift.jpg')},
 {name: 'Walls', image: Rails.root.join('/app/assets/images/categories/Walls.jpg')},
 {name: 'Whales & Sharks', image: Rails.root.join('/app/assets/images/categories/Sharks.jpg')},
 {name: 'Macro/Corals', image: Rails.root.join('/app/assets/images/categories/Corals.jpg')},
 {name: 'Manta Rays', image:Rails.root.join('/app/assets/images/categories/Manta.jpg')},
 {name: 'Night', image: Rails.root.join('/app/assets/images/categories/Night.jpg')},
 {name: 'Turtles', image: Rails.root.join('/app/assets/images/categories/Turtles.jpg')},
 {name: 'Schools of Fish', image: Rails.root.join('/app/assets/images/categories/Fish.jpg')}]

category_attributes.each do |attributes|
  Category.create(attributes)
end

month_attributes = [{name: 'January'}, {name: 'February'},
  {name: 'March'}, {name: 'April'}, {name: 'May'},
  {name: 'June'}, {name: 'July'}, {name: 'August'},
  {name: 'September'}, {name: 'October'}, {name: 'November'},
  {name: 'December'}]

month_attributes.each do |attributes|
  Month.create(attributes)
end

you want to do

category_attributes.each do |category|
  Category.find_or_create_by(category)
end

same with the months. use find or create by