An association from the table X refers to an unmapped class:

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C#

NHibernate

Fluent

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I have several tables. I 'm using nhibernate in my desktop project. But when I try to save some data into my any of table through nhibernate it gives me following error

{"An association from the table p_returnitem refers to an unmapped class: SmartWarehouse.Order"}

My Preturnitementity is following

class PReturnitemEntity : Entity
{
    //public virtual int id { get; set; }
    public virtual string purchase_items { get; set; }
    public virtual double return_sum { get; set; }
    public virtual double rate { get; set; }
    public virtual int r_count { get; set; }
    public virtual bool deleted { get; set; }
    public virtual DateTime create_date { get; set; }
    public virtual DateTime update_date { get; set; }

    public virtual UsersEntity user { get; set; }
    public virtual PurchaseEntity purchase { get; set; }
    public virtual CurrencyEntity currency { get; set; }


}

Preturitem Mapping Class

class PreturnItemMap : ClassMap<PReturnitemEntity>
   {
       public PreturnItemMap()
       {
           Id(x => x.Id);
           Map(x => x.purchase_items);
           Map(x => x.return_sum);
           Map(x => x.rate);
           Map(x => x.r_count);

           Map(x => x.deleted);
           Map(x => x.create_date);
           Map(x => x.update_date);

           References(x => x.user).ForeignKey("user_id").Cascade.None();
           References(x => x.currency,"cur_code").PropertyRef(x => x.code).Cascade.None();
           References(x => x.purchase,"purchase_id").Cascade.None();

           Table("p_returnitem");
       }
   }

what might be wrong in here please suggest to me what to do

What I have tried:

I checked all entity class and all mapping class double check everything is connected well