public class Assent
    {
        /*0)*/ public virtual string Id { get; set; }
        /*1)*/ public virtual Enterprise Enterprise { get; set; }
        /*2)*/ public virtual string WaterObjectName { get; set; }
        /*3)*/ public virtual IList<WaterUsePurposeSet> WaterUsePurposes { get; set; }
        /*4)*/ public virtual IList<WaterSourceSet> WaterSources { get; set; }
        /*5)*/ public virtual string WaterIntake { get; set; }
        /*6)*/ public virtual IList<Well> Wells { get; set; }
        /*7)*/ public virtual string FishProtectionConstructions { get; set; }
        /*8)*/ public virtual string SanitaryControl { get; set; }
        /*9)*/ public virtual IList<WaterConsumptionIndicatorSet> WaterConsumptionIndicatorSets { get; set; }
       /*10)*/ public virtual IList<WasteWaterIndicatorSet> WasteWaterIndicatorSets { get; set; }
       /*11)*/ public virtual string WaterUseRange { get; set; }
       /*12)*/ public virtual IList<OutletSet> OutletSets { get; set; }
       /*13)*/ public virtual string WasteWaterCleaning { get; set; }
       /*14)*/ public virtual IList<AccountingModeSet> AccountingModeSets { get; set; }
       /*15)*/ public virtual IList<Activity> Activities { get; set; }
       /*16)*/ public virtual IList<Condition> OtherConditions { get; set; }
       /*17)*/ public virtual IList<Agreement> Agreements { get; set; }
       /*18)*/ public virtual DateTime IssueDate { get; set; }
       /*19)*/ public virtual DateTime ValidityFromDate { get; set; }
       /*20)*/ public virtual DateTime ValidityToDate { get; set; }
       /*21)*/ public virtual string AssentNumber { get; set; }
       /*22)*/ public virtual IList<Document> Documents { get; set; }
    }

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"
                   assembly="dt_shared"
                   namespace="dt_shared">
  <class name="dt_shared.Assent" lazy="false" table ="Assents">
    <id name="Id">
      <column name="AssentId" sql-type="int"/>
      <generator class="native"/>
    </id>
    <property name="WaterObjectName"  column="WaterObjectName"/>
    <property name="WaterIntake"      column="WaterIntake"/>
    <property name="FishProtectionConstructions" column="FishProtectionConstructions"/>
    <property name="SanitaryControl"  column="SanitaryControl"/>
    <property name="WaterUseRange"    column="WaterUseRange"/>
    <property name="WasteWaterCleaning" column="WasteWaterCleaning"/>
    <property name="AssentNumber"     column="AssentNumber"/>
    <property name="IssueDate"        column="IssueDate"/>
    <property name="ValidityFromDate" column="ValidityFromDate"/>
    <property name="ValidityToDate"   column="ValidityToDate"/>

    <many-to-one name="Enterprise" column="EnterpriseId"/>
    
    <bag name ="WaterUsePurposes" table="Assents_WaterusePurposes"  cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="WaterUsePurposeSet"/>
    </bag>

    <bag name="WaterSources" table="Assents_WaterSources" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="WaterSourceSet"/>
    </bag>

    <bag name="Wells" table="Wells" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="Well"/>
    </bag>

    <bag name="WaterConsumptionIndicatorSets" table="WaterConsumptionIndicators_Values" cascade="all-delete-orphan" lazy="false" >
      <key column="AssentId" not-null="true"/>
      <one-to-many class="WaterConsumptionIndicatorSet"/>
    </bag>
    
    <bag name="WasteWaterIndicatorSets" table="WastewaterIndocators_Values" cascade="all-delete-orphan" lazy="false" >
      <key column="AssentId" not-null="true"/>
      <one-to-many class="WasteWaterIndicatorSet"/>
    </bag>

    <bag name="OutletSets" table="Assent_Outlets" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="OutletSet"/>
    </bag>

    <bag name="AccountingModeSets" table="Sources_AccountingModes" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="AccountingModeSet"/>
    </bag>

    <bag name="Activities" table="Activities" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="Activity"/>
    </bag>

    <bag name="OtherConditions" table="Assents_Conditions" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="Condition"/>
    </bag>
    
    <bag name="Agreements" table="Agreements" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="Agreement"/>
    </bag>

    <bag name="Documents" table="Assents_Documents" cascade="all-delete-orphan" lazy="false">
      <key column="AssentId" not-null="true"/>
      <one-to-many class="Document"/>
    </bag>
    
  </class>
</hibernate-mapping>

    public class Proxy
    {
        public Assent RegisterAssent(Assent assent)
        {
            if (assent != null)
                using (var session = GetSessionFactory().OpenSession())
                {
                    using (var tx = session.BeginTransaction())
                    {
                        try
                        {
                            if (assent.Enterprise != null)
                            {
                                var retrievedEnterprise = RegisterEnterprise(assent.Enterprise);
                                assent.Enterprise = retrievedEnterprise;
                                assent = session.Merge<Assent>(assent); // тут вываливается исключение
                                tx.Commit();
                            }
                        }
                        catch (Exception ex)
                        {
                            throw new Exception("Помилка при збереженні дозволу");
                        }
                    }
                }
            return assent;
        }
    }

public Assent RegisterAssent(Assent assent)
        {
            Assent persistedAssent = null;
            if (assent != null)
            {
                Assent assentToPersist = assent.CreateCopy(); // создается полная копия объекта
                if (assentToPersist != null)
                {
                    try
                    {
                        using (var session = GetSessionFactoryFluently().OpenSession())
                        using (var tx = session.BeginTransaction())
                        {
                            if (assentToPersist.Enterprise != null)
                            {
                                session.SaveOrUpdate(assentToPersist.Enterprise); 
                                if (assentToPersist.Enterprise.Id != 0)
                                {
                                    session.SaveOrUpdate(assentToPersist); // тут сохраняются добавленные элементы коллекций
                                    tx.Commit();
                                }
                            }
                        }
                        using (var session = GetSessionFactoryFluently().OpenSession())
                        using (var tx = session.BeginTransaction())
                        {
                            persistedAssent = session.Merge<Assent>(assent); // тут удаляются из БД удаленные элементы коллекций
                            tx.Commit();
                        }
                    }
                    catch (Exception ex)
                    {
                        throw new Exception("Помилка при збереженні дозволу");
                    }
                }
            }
            return persistedAssent;
        }