Parse Cloud update unique column

问题

I have used this code to save the data using Parse Cloud, I have a unique column "number_plate" I am facing a problem and that is when I try to update the object I'm unable to do so.

var Car = Parse.Object.extend("Car");

// Check if number plate is set, and enforce uniqueness based on the number plate column.
Parse.Cloud.beforeSave("Car", function(request, response) {
  if (!request.object.get("number_plate")) {
    response.error('The number plate was not provided.');
  } else {
    var query = new Parse.Query(Car);
    query.equalTo("number_plate", request.object.get("number_plate"));
    query.first({
      success: function(object) {
        if (object) {
          response.error("A Car with this number plate already exists.");
        } else {
          response.success();
        }
      },
      error: function(error) {
        response.error("Could not validate uniqueness for this Car because the number plate is repeated.");
      }
    });
  }
});

**

UPDATE: The new code I used according to the given answer:

**

Parse.Cloud.beforeSave("Car", function(request, response) {
    if (!request.object.isNew()) {
      // Let existing object updates go through
      response.success();
    }
    var query = new Parse.Query("Car");
    // Add query filters to check for uniqueness
    query.equalTo("number_plate", request.object.get("number_plate"));
    query.first().then(function(existingObject) {
      if (existingObject) {
        // Update existing object. here you can do all the object updates you want 
        if (request.object.get("car_model") != undefined){
          object.set("car_model",request.object.get("car_model"));
        }

        if (request.object.get("car_name") != undefined){
          object.set("car_name",request.object.get("car_name"));
        }
        if (request.object.get("year") != undefined){
          object.set("year",request.object.get("year"));
        }
        return existingObject.save();
        response.error();
      } else {
        // Pass a flag that this is not an existing object
        return Parse.Promise.as(false);
      }
    }).then(function(existingObject) {
      if (existingObject) {
        // Existing object, stop initial save
        response.error("Existing object");
      } else {
        // New object, let the save go through
        response.success();
      }
    }, function (error) {
      response.error(error);
    });
});

回答1:

The key in this case is to work with the promises.

What you are trying to reach is something like the following:

Parse.Cloud.beforeSave("Car", function(request, response) {
    if (!request.object.isNew()) {
      // Let existing object updates go through
      response.success();
    }
    var query = new Parse.Query("Car");
    // Add query filters to check for uniqueness
    query.equalTo("number_plate", request.object.get("number_plate"));
    query.first().then(function(existingObject) {
      if (existingObject) {
        // Update existing object. here you can do all the object updates you want 
        if (request.object.get("columnToUpdate") != undefined){
          existingObject.set('columnToUpdate',request.object.get("columnToUpdate"));
        }
        return existingObject.save();
        response.error();
      } else {
        // Pass a flag that this is not an existing object
        return Parse.Promise.as(false);
      }
    }).then(function(existingObject) {
      if (existingObject) {
        // Existing object, stop initial save
        response.error("Existing object");
      } else {
        // New object, let the save go through
        response.success();
      }
    }, function (error) {
      response.error(error);
    });
});

来源：https://stackoverflow.com/questions/34499751/parse-cloud-update-unique-column