Try to add tests even though I don't know how to do that...

spec/lib/search_query_transformer_spec.rb

@@ -15,4 +15,58 @@
       expect(transformer.filter_clauses.first).to be_nil
     end
   end
+
+  describe '#apply' do
+    subject(:applied_query) do
+      described_class.new(
+        SearchQueryParser.new.parse('query')
+      ).apply(query)
+    end
+
+    let(:query) { { bool: {} } }
+    let(:search_string) { 'search string' }
+    let(:term) { { term: { field_name: search_string } } }
+
+    context 'when query is just a bool' do
+      it 'does not modify the query' do
+        expect(applied_query).to eq(query)
+      end
+    end
+
+    context 'when should_clauses are present' do
+      let(:should) { { bool: { should: [term] } } }
+
+      it 'adds should clauses to the query' do
+        expect(applied_query[:bool][:should]).to include(term)
+      end
+
+      it 'sets minimum_should_match to 1' do
+        expect(applied_query[:minimum_should_match]).to eq(1)
+      end
+    end
+
+    context 'when must_clauses are present' do
+      let(:must) { { bool: { must: [term] } } }
+
+      it 'adds must clauses to the query' do
+        expect(applied_query[:bool][:must]).to include(term)
+      end
+    end
+
+    context 'when must_not_clauses are present' do
+      let(:must_not) { { bool: { must_not: [term] } } }
+
+      it 'adds must_not clauses to the query' do
+        expect(applied_query[:bool][:must_not]).to include(term)
+      end
+    end
+
+    context 'when filter_clauses are present' do
+      let(:filter) { { bool: { filter: [term] } } }
+
+      it 'adds filter clauses to the query' do
+        expect(applied_query[:bool][:filter]).to include(term)
+      end
+    end
+  end
 end