Fix calculating tsvector on hidden records

We introduced this bug in commit 55d339572, since we didn't take hidden
records into consideration.

I've tried to use `update_column` to simplify the code, but got a syntax
error `unnamed portal parameter` and didn't find how to fix it.

app/models/concerns/search_cache.rb

@@ -6,7 +6,7 @@ module SearchCache
   end
 
   def calculate_tsvector
-    self.class.where(id: id).update_all("tsv = (#{searchable_values_sql})")
+    self.class.with_hidden.where(id: id).update_all("tsv = (#{searchable_values_sql})")
   end
 
   private

spec/models/search_cache_spec.rb

@@ -0,0 +1,27 @@
+require "rails_helper"
+
+describe SearchCache do
+  describe "#calculate_tsvector" do
+    it "calculates the tsv column of a record" do
+      debate = create(:debate)
+      debate.update_column(:tsv, nil)
+
+      expect(debate.reload.tsv).to be_nil
+
+      debate.calculate_tsvector
+
+      expect(debate.reload.tsv).not_to be_nil
+    end
+
+    it "calculates the tsv column of a hidden record" do
+      debate = create(:debate, :hidden)
+      debate.update_column(:tsv, nil)
+
+      expect(debate.reload.tsv).to be_nil
+
+      debate.calculate_tsvector
+
+      expect(debate.reload.tsv).not_to be_nil
+    end
+  end
+end