db={"users":[{"_id":1,"associatedTenants":[{"tenantId":"A","role":"foo"},{"tenantId":"B","role":"bar"}]}],"tenants":[{"_id":"A","name":"Lorem"}]}

db.users.aggregate([{"$lookup":{"from":"tenants","localField":"associatedTenants.tenantId","foreignField":"_id","as":"tenants"}},{"$addFields":{"associatedTenants":{"$map":{"input":"$associatedTenants","in":{"$mergeObjects":["$$this",{"tenant":{"name":{"$arrayElemAt":["$tenants.name",{"$indexOfArray":["$tenants._id","$$this.tenantId"]}]}}}]}}},"tenants":"$$REMOVE"}}])