title: Mongodb spring data group _id toLower or case insensitive
body: <p>I have mongodb facet and aggregate query in spring boot using mongo template and aggregation query. everything works fine except case sensitivity of value. I have below query in mongodb:</p>
  
  <pre><code>db.getCollection('product').aggregate([{"colors":[{$unwind:"$variants"},
  {"$group": {
          _id: { $toLower: "$variants.color" },
          count:{$sum:1},
          image : { $first: '$variants.color_image' },
      }}
  
  ]
  </code></pre>
  
  <p>I have equivalent spring data query:</p>
  
  <pre><code>Aggregation.facet(unwind("variants"), group("variants.color").count().as("count").first("variants.color_image").as("image"))
              .as("colors");
  </code></pre>
  
  <p>But here how can I mention toLower to group field?</p>