update or add a property to subdocuments of all found documents (MongoDB)

This updateMany finds all agreement_versions that has a participants property on it and sets each member (the $[] selects each individual member) of participants to have a field called role with a value of "Decider".

db.agreement_versions.updateMany(
  {
    participants: {
      $exists: true,
    },
  },
  {
    $set: {
      "participants.$[].role": "Decider",
    },
  }
);