Mongodb $project: $filter sub-array

Question

There is an items (mongoose) schema that looks like this (simplified to what it matters to the question):

{
    brand: {
        name: String,
    },
    title: String,
    description: [{ lang: String, text: String }],
    shortDescription: [{ lang: String, text: String }],
    variants: {
        cnt: Number,
        attrs: [
            {
                displayType: String,
                displayContent: String,
                displayName: [{ lang: String, text: String }],
                name: String,
            },
        ],
    }
}

I'm trying to filter the items by language, so I've constructed the following query:

db.items.aggregate([
    { $match: { 'description.lang': 'ca', 'shortDescription.lang': 'ca' } },
    { $project: {
        'brand.name': 1,
        title: 1,
        description: {
            '$filter': {
                input: '$description',
                as: 'description',
                cond: { $eq: ['$$description.lang', 'ca'] }
            }
        },
        shortDescription: {
            '$filter': {
                input: '$shortDescription',
                as: 'shortDescription',
                cond: { $eq: ['$$shortDescription.lang', 'ca'] }
            }
        },
        'variants.cnt': 1,
        'variants.attrs': 1
    } }
])

And it works as expected: it filters description and shortDescription by language. Right now I'm wondering if it could be possible to filter every variants.attrs.$.displayName as well. Is there any way to do it?

I've been trying to $unwind variant.attrs but I get completly lost when trying to $group again and I'm not really sure if this is the best way...

Answer 1

You are nearly there. Try these steps:

• use $unwind stage before $project stage to expand the outer array of documents, i.e. variants.attrs
• Add filter for the sub array variants.attrs.displayName in the $project stage.
• You will have to project all the sub fields of variants key.
• Next add $group stage and group by all the elements except the sub-array. Use $push to rebuild the sub array in group by stage.

• Lastly, add $project stage to rebuild the document to its original structure.

  db.items.aggregate([
    { $match: { 'description.lang': 'ca', 'shortDescription.lang': 'ca' } },
    { $unwind : "$variants.attrs" },
    { $project: {
       '_id' : 1,
       'brand.name': 1,
       title: 1,
       description: {
          '$filter': {
              input: '$description',
              as: 'description',
              cond: { $eq: ['$$description.lang', 'ca'] }
          }
       },
       shortDescription: {
         '$filter': {
             input: '$shortDescription',
             as: 'shortDescription',
             cond: { $eq: ['$$shortDescription.lang', 'ca'] }
          }
       },
       'variants.attrs.displayName' : {
          '$filter' : {
             input: '$variants.attrs.displayName',
             as: 'variants_attrs_displayName',
             cond: { $eq : ['$$variants_attrs_displayName.lang','ca']}
          }
        },
  
        'variants.cnt': 1,
        'variants.attrs.displayType': 1,
        'variants.attrs.displayContent' : 1,
        'variants.attrs.name' : 1
      } 
   } , { $group:
            {
              _id : { 
                  _id: "$_id",
                  title: "$title",
                  brand:"$brand",
                  description:"$description",
                  shortDescription:"$shortDescription", 
                  variants_cnt : "$variants.cnt"
                  },
              variants_attrs : { $push : 
              { 
                displayType : "$variants.attrs.displayType",
                displayContent : "$variants.attrs.displayContent",
                displayName : "$variants.attrs.displayName",
                name: "$variants.attrs.name" 
              }
            }
          }
      },
  { $project : 
   {
      "_id" : 0,
      brand : "$_id.brand",
      title : "$_id.title",
      description : "$_id.description",
      shortDescription : "$_id.shortDescription",
      variants : {
        cnt : "$_id.variants_cnt" ,
        attrs : "$variants_attrs"
       }
     }  
   }
  ])
  

Depending on your use case, you should reconsider your data model design to avoid duplication of filter values. i.e. 'description.lang': 'ca', 'shortDescription.lang': 'ca', 'variants.attrs.displayName.lang': 'ca'