Add basic post creation test

test/application_system_test_case.rb

@@ -62,4 +62,42 @@ class ApplicationSystemTestCase < ActionDispatch::SystemTestCase
       users(user_or_fixture)
     end
   end
+
+  # In the post form, this method will select the given tag.
+  #
+  # @param tag_name [String] the name of the tag
+  # @param create_new [Boolean] whether creating a new tag is allowed (default false)
+  def post_form_select_tag(tag_name, create_new = false)
+    # First enter the tag name into the select2 search field for the tag
+    within find_field('Tags (at least one):').find(:xpath, '..') do
+      find('.select2-search__field').fill_in(with: tag_name)
+    end
+
+    # Get the first item listed that is not the "Searching..." item
+    first_option = find('#select2-post_tags_cache-results li:first-child') { |el| el.text != 'Searching…' }
+
+    if first_option.first('span').text == tag_name
+      # If the text matches the tag name, first check whether we are creating a new tag.
+      # If so, confirm that we are allowed to. If all is good, actually click on the item.
+      if create_new || !first_option.text.include?('Create new tag')
+        first_option.click
+      else
+        raise "Expected to find tag with the name #{tag_name}, " \
+              'but could not select it from options without creating a new tag.'
+      end
+    elsif create_new
+      # The first item returned is not the tag we were looking for (another tag partial match + not existing)
+      # If we are allowed to create a tag, select the last option from the list, which is always the tag creation.
+      last_option = find('#select2-post_tags_cache-results li:last-child')
+      if last_option.first('span').text == tag_name
+        last_option.click
+      else
+        raise "Tried to select tag #{tag_name} for creation, but it does not seem to be a presented option."
+      end
+    else
+      # The first item returned is not the tag we were looking for, and we are not allowed to create a tag.
+      raise "Expected to find tag with the name #{tag_name}, " \
+            'but could not select it from options without creating a new tag.'
+    end
+  end
 end

test/system/post_test.rb

+require 'application_system_test_case'
+
+class PostTest < ApplicationSystemTestCase
+  test 'Not-signed in user cannot create a post' do
+    visit root_url
+    click_on 'Create Post'
+
+    assert_current_path new_user_session_url
+  end
+
+  test 'Signed in user can create a post' do
+    category = categories(:meta)
+    log_in :standard_user
+    visit category_path(category)
+    click_on 'Create Post'
+
+    fill_in 'Body', with: "When running QPixel, users are generally supposed to be able to create posts.\n" \
+                          'Does that actually work?'
+    fill_in 'Summarize your post with a title:', with: 'Can a signed-in user create a post?'
+    post_form_select_tag tags(:faq).name
+    click_on "Save Post in #{category.name}a"
+  end
+
+  # TODO: Post validations
+
+  # TODO: Sort urls
+end
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