db={processes:[{_id:"p1",steps:[{_id:"ps1",step:"s1",time:10},{_id:"ps2",step:"s2",time:15}]}],steps:[{_id:"s1",name:"step 1"}]}

db.processes.aggregate([{$lookup:{from:"steps",let:{stepId:"$steps.step"},pipeline:[{$match:{$expr:{$in:["$_id","$$stepId"]},}}],as:"steps",}}])