多表联查

$match + $lookup

https://www.mongodb.com/docs/manual/reference/operator/aggregation/lookup/

{
   $lookup:
     {
       from: <collection to join>,
       localField: <field from the input documents>,
       foreignField: <field from the documents of the "from" collection>,
       as: <output array field>
     }
}

示例

db.getCollection('var').aggregate([
  {$match:  { 'Variation ID': 'RC12' } },
  {$lookup: {
    from: 'var_detail',
    localField: 'Variation ID',
    foreignField: 'Variation ID',
    as: 'detail' }
  },
])