Return error if you try to generate a model that already exists (#1127)

* return an error message if you try to generate a model that already exists. Fixes #1008

* Update tasks/gen/model.cr

Co-Authored-By: Paul Smith <paulcsmith@users.noreply.github.com>

* fixing failing specs

Co-authored-by: Paul Smith <paulcsmith@users.noreply.github.com>

spec/tasks/gen/model_spec.cr

@@ -86,4 +86,19 @@ describe Gen::Model do
       io.to_s.should contain("Model name should only contain letters")
     end
   end
+
+  it "displays an error if the model has already been generated" do
+    with_cleanup do
+      ARGV.push("User")
+
+      Gen::Migration.silence_output do
+        io = IO::Memory.new
+        Gen::Model.new.call(io)
+      end
+
+      io = IO::Memory.new
+      Gen::Model.new.call(io)
+      io.to_s.should contain("'User' model already exists at ./src/models/user.cr")
+    end
+  end
 end

tasks/gen/model.cr

@@ -35,7 +35,8 @@ class Gen::Model < LuckyCli::Task
     resource_name_is_present &&
       resource_name_is_camelcase &&
       resource_name_matches_format &&
-      columns_are_supported
+      columns_are_supported &&
+      resource_name_not_taken
   end
 
   private def resource_name_is_present
@@ -59,6 +60,11 @@ class Gen::Model < LuckyCli::Task
     columns_are_valid?
   end
 
+  private def resource_name_not_taken
+    @error = "'#{resource_name.camelcase}' model already exists at #{"./src/models/#{template.underscored_name}.cr"}."
+    !File.exists?("./src/models/#{template.underscored_name}.cr")
+  end
+
   private def template
     Lucky::ModelTemplate.new(resource_name, columns)
   end