eelcoj

Ah, another typo—though you got it working, but for maintainability—a (possible) solution. @projects_id returns an object, while it needs an array. That line should be something like this: @projects_id = @projects.find(:project_id).pluck(:id) (which should return [1, 2, 3].

While above solution works, looks like you had a typo here:
@tasks = Project.where(:project_id => @projects_id).all

instead of @tasks = Task.where(:project_id => @projects_id).all

Some actual dev advice (dev platform and all): stick to Devise for authentication, don't waste your time bulding your own. Devise is tested throughout the years and the many gotchas you stumble upon are covered by using Devise. Mixed dev/bizz: stick to pure web developement for now, it's overwhelming enough already.

I can give you diferent types of advice (dev or bizz), going for the latter as that's what I would've wanted 10 years ago in this situation.
Don't overthink this. If you are working on your first app (assuming based on your post), don't even think about mobile now (nor Vue).
Think about what you want your product to really be and focus on just that and really nothing more.

More bizz-advice: do not build anything more than a simple blog where you can build an audience first based on useful articles about gin.

You are currently using a form_with form helper and passing a model (@article). You can see in the actual source the link this creates. You can also pass an url to form_with, eg form_with url: admin_articles_path, method: :post do |f|. Which should do the "trick" here.

Are using the correct url? So instead of blabla_path use blabla_url?

For 1. have a look at many-to-many relationship. With Rails this can be done like follows:
Sport.rb
has_many: :country_sports
has_many: sports, through: :country_sports
Country.rb
has_many :country_sports
has_many :sports, through: :country_sports
CountrySport.rb
belongs_to :sport
belongs_to :country
The last model is the one that connects the Sport and Country model and for the bare minimum only need country_id and sport_id.

For 2, you can nest the routes like follows:
resources :sports do
resources :country do
resources :leagues do
resources :matches
end
end
end

This is the gist of it, so feel free if you have specific issues with this.

From a quick glance all looks good. Any error? Does the log give you anything?

This is where a service object comes into play. This is a so called “Plain Old Ruby Object (PORO)” where you can put in specific code for one "job". You can then call this object from anywhere within your app (controller, other service object, etc.).
For me to good wrap my head around I searched from some howto's, read them, noted the best practices around them (eg. only one public method) and applied it to my own app.

Not sure if there is an episode about service objects on GoRails.

Your syntax seems to be off. Try something like this:

f.select :id, options_from_collection_for_select(@select, :id, :name)

cannot load such file -- sqlite3/sqlite3_native (LoadError)

This seems to be your best bet. Did you have sqlite installed?

Easiest way is to use a view helper and put the logic in there.

Something along the lines of:

def user_avatar(user_id)
    user = User.find!(user_id)
    if user.avatar.attached?
        image_tag user.avatar
    else
        image_tag 'default_avatar.jpg'
    end
end

web:    bundle exec thin start -p $PORT

worker: bundle exec rake resque:work QUEUE=*