custom validation doesn't prevent the create action from posting

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C#

MVC

validation

I have a custom validation that seems to work fine, it get hit and the logic is working but when IsValid return false it still go to the create post method

here is my custom validation class

public class CheckIfSparesAvailableAttribute : ValidationAttribute
    {
        public CheckIfSparesAvailableAttribute() : base() { }

        public override bool IsValid(object value)
        {
            int quantity, spareID = (int)value;

            quantity = new Spares().CheckQuantity(spareID);

            if (quantity < 1)
                return false;
            return true;
            
        }
    }

and here is my model

public class IncidentSpares
    {
        //other properties
        [Required]
        [Display(Name="Spare")]
        [CheckIfSparesAvailable(ErrorMessage="Test")]
        public int SpareID { get; set; }
    }

PS: this is a many-to-many table and am trying to validate the Incident class which contain ICollection of IncidentSpare
here is the Incident model

public class Incidents
    {
        //other properties
        [Display(Name="Spares")]
        public virtual ICollection<IncidentSpares> SpareParts { get; set; }
    }

the create method am talking about is in the incidentController

[update]
am trying to make this work on client side I've written GetClientValidationRules method as follows

public IEnumerable<ModelClientValidationRule> GetClientValidationRules(ModelMetadata metadata, ControllerContext context)
       {
           var rule = new ModelClientValidationRule
           {
               ValidationType = "checkifsparesavailable",
               ErrorMessage = "Spare is not available in stock",
           };
           yield return rule;
       }

do i have to write javascript to make this work in client side, if yes plz help