Example query:

 mutation{
  insert_Company(objects: [{
    id: 36, description: "test"
  }], on_conflict: { action: update  ,
    constraint: Company_pkey  }){
affected_rows
  }
}  

returned error:

{"errors":[{"path":"$","error":"constraint "company_pkey" for table "Company" does not exist","code":"constraint-error"}]}

according to @0x777: "When generating the sql, we don't seem to be quoting the constraint name
and because of how sql treats case not quoting Company_pkey is causing problems."