db={"sites":[{"_id":"5acdb8f65ea63a27c1facf86","TemplateId":"sdfsdfs34234234sdf"}],"fields":[{"_id":"jsd32423423423","Type":"Phone","Name":"Phone_Test"},{"_id":"2342sdffafdasdfdfs","Type":"Numeric","Name":"Number_Test"}],"templates":[{"_id":"sdfsdfs34234234sdf","Type":"Site","Name":"Site 1","Sections":[{"id":"8asdf89asd8f9sdf","Header":"Header1","FieldItems":[{"FieldId":"jsd32423423423","x":1,"y":0},{"FieldId":"2342sdffafdasdfdfs","x":1,"y":1}]}]}]}

db.sites.aggregate([{$lookup:{from:"templates","let":{template_id:"$TemplateId"},pipeline:[{$match:{$expr:{$eq:["$_id","$$template_id"]}}},{$unwind:"$Sections"},{$unwind:"$Sections.FieldItems"},{$lookup:{from:"fields","let":{field_id:"$Sections.FieldItems.FieldId"},pipeline:[{$match:{$expr:{$eq:["$_id","$$field_id"]}}}],as:"Sections.FieldItems.Fields"}},{$group:{_id:{secId:"$_id",fId:"$Sections.id"},Type:{$first:"$Type"},Name:{$first:"$Name"},Header:{$first:"$Sections.Header"},fieldItems:{$push:"$Sections.FieldItems"}}},{$group:{_id:"$_id.secId",Type:{$first:"$Type"},Name:{$first:"$Name"},Sections:{$push:{id:"$_id.fId",Header:"$Header",fieldItems:"$fieldItems"}}}}],as:"Templates"}}])