AsyncResultPreventSave.JS

var CourseForm = (function () {

    var SaveMode = {
			Save: 1,
			SaveAndClose: 2,
			SaveAndNew: 59,
			Autosave: 70
    };

    //Variable that shows if validation was successfully passed or not
    var isValidationNeeded = true;

    function OnSave(executionContext) {
        //so if there are several save handlers and one of previous already called preventDefault
        //there is no need to do any validations anymore
        if (executionContext.getEventArgs().isDefaultPrevented()) {
            return;
        }

        //getting save mode from event
        var saveMode = executionContext.getEventArgs().getSaveMode();

        //if savemode is not one of listed - just quit the execution and let the record to be saved
        if (saveMode !== SaveMode.Save &&
            saveMode !== SaveMode.SaveAndClose &&
            saveMode !== SaveMode.SaveAndNew &&
            saveMode !== SaveMode.Autosave) {
            return;
        }

        //so if validation was successfully passed - flag is reset
        //and code just leaves the form alone and allows changes to be saved
        if (!isValidationNeeded) {
            isValidationNeeded = true;
            return;
        }

        //getting of the form context from execution context object
        var formContext = executionContext.getFormContext();
        
		var student = formContext.getAttribute("soft_student").getValue()[0].id;
        var course = formContext.getAttribute("soft_course").getValue()[0].id;
        var coursestartdate = formContext.getAttribute("soft_coursestartdate").getValue();

        if (student == "" || course == "" || coursestartdate == null) {
            return;
        }

        //preventing of the save operation before async operation is started
        executionContext.getEventArgs().preventDefault();

        var formatteddate = coursestartdate.getFullYear().toString() + "-" + (coursestartdate.getMonth() + 1).toString() + "-" + coursestartdate.getDate().toString();
        var fetchXML = "?fetchXml=<fetch mapping='logical'><entity name='soft_studentregisteredcourses'><attribute name='soft_name' /><order attribute='soft_name' descending='false' /><filter type='and'><condition attribute='soft_coursestartdate' operator='on' value='" + formatteddate + "' /><condition attribute='soft_course' operator='eq' uitype='soft_course' value='" + course + "' /><condition attribute='soft_student' operator='eq' uitype='soft_student' value='" + student + "' /><condition attribute='statecode' operator='eq' value='0' /></filter></entity></fetch>";

        Xrm.WebApi.retrieveMultipleRecords("soft_studentregisteredcourses", fetchXML).then(
            function success(results) {
                //so if there are other records with the same student and course with course start date
                if (results.entities.length !== 0) {
                    //this message is shown to user only when user caused save, autosave is just blocked
                    if (saveMode !== SaveMode.Autosave) {
                        var recordURL = GetEntityRecordUrl(executionContext, false, true, results.entities[0].soft_studentregisteredcoursesid);

                        var entityName = executionContext.getFormContext().data.entity.getEntityName();
                        var entityId = results.entities[0].soft_studentregisteredcourseid;

                        //var data = {
                        //EntityName: entityName,
                            //EntityId: entityId
						//};

						//custom parameter that you need in the modal dialog 
						var dialogParameters = {
							pageType: "webresource",//required 
							webresourceName: "soft_rewardvalidatehtml.html",//Html Webresource that will be shown
							data:recordURL //optional
                       
						};

						var navigationOptions = {
							target: 2,//use 1 if you want to open page inline or 2 to open it as dialog 
							width: 400,
							height: 300,
							position: 1,//1 to locate dialog in center and 2 to locate it on the side,
							title: "Validation Error"
                        };

                        Xrm.Navigation.navigateTo(dialogParameters, navigationOptions).then(
                        function (returnValue) {
                            //returnValue is blank when "Cancel" button is clicked 
                            if (!returnValue) {
                                return;
                            }

                            console.log(returnValue);

                            //Add your processing logic here    
                        },
                        function (e) {
                            //put your error handler here 
                        });

					}
				} 
				else {
					//otherwise validation flag is set to "Passed"
					isValidationNeeded = false;
					//and save event is called again
					if(saveMode === SaveMode.Save ||
							saveMode === SaveMode.Autosave) {
						formContext.data.entity.save();
					} 
					else if (saveMode === SaveMode.SaveAndClose) {
						formContext.data.entity.save("saveandclose");
					} 
					else {
						formContext.data.entity.save("saveandnew");
					}
				}
			},
			function (error) {
			//if something went wrong - error message is shown to user
			Xrm.Navigation.openAlertDialog({ text: error.message });
			}
		);
    }

	function GetEntityRecordUrl(executionContext, forceClassic = false, newWindow = true, recordId) {
		var strUIType = 'forceUCI=1';
		if (forceClassic == true || forceClassic == 1)
			strUIType = 'forceClassic=1';

		if (newWindow == 0)
			newWindow = false;

		var entityName = executionContext.getFormContext().data.entity.getEntityName();
		var entityId = recordId;

		var entityRecordUrl = executionContext.getContext().getClientUrl() + '/main.aspx?' + strUIType;
		entityRecordUrl += '&newWindow=' + newWindow;
		entityRecordUrl += '&pagetype=entityrecord&etn=' + entityName + '&id=' + entityId;

		return entityRecordUrl;
	}

	return {
		OnSave: OnSave
	};
	
})();