public virtual IEnumerable<Contract> Contracts // для внешних слоев приложения
        {
            get { return new ImmutableSet<Contract>(_contracts); }
        }
        protected internal virtual Iesi.Collections.Generic.ISet<Contract> _contracts { get; set; } // NH, entities

      <set cascade="save-update" inverse="true" name="_contracts" where="DeletionDate is null" mutable="true">
        <key foreign-key="FK_ТакаяТоOrganization_Contract_ТакаяТоOrganizationId">
          <column name="ТакаяТоOrganizationId" />
        </key>
        <one-to-many class="..." />
      </set>

такаяТоOrganization._contracts.Add(this);

class A {
  void DeleteB(B b)
  {
    collection_of_b.Remove(b);
    if (b.Collection_of_a.Any(x => x.Id == this.Id)) // =================== В этом вся суть. Как вам такое??
    {
      b.DeleteA(this);
    }
  }
  public Collection_of_b ... // read-only, для всех
  private collection_of_b ... // для себя, можно менять
}

class B {
  void DeleteB(A a)
  {
    collection_of_a.Remove(a);
    if (a.Collection_of_b.Any(x => x.Id == this.Id)) // =================== В этом вся суть. Как вам такое??
    {
      a.DeleteB(this);
    }
  }
  public Collection_of_a ... // read-only, для всех
  private collection_of_a ... // для себя, можно менять
}

        protected internal virtual void AddContract(Contract contract)
        {
            if (contract.Id != 0 && _contracts.Any(x => x.Id == contract.Id))
            {
                throw new Exception("Данный договор уже связан с этой организацией.");
            }

            _contracts.Add(contract); ///////////////
        }

<property name="Actor" not-null="true" />

            References(x => x.ТакаяТоOrganization).Column("ТакаяТоOrganizationId").Not.Nullable();

    <many-to-one class="ТакаяТоOrganization, ... foreign-key="FK_Contract_ТакаяТоOrganization" name="ТакаяТоOrganization">
      <column name="ТакаяТоOrganizationId" not-null="true" />
    </many-to-one>

<list name="Actors" cascade="all-delete-orphan">
  <key column="MovieId" />
  <index column="ActorIndex" />
  <one-to-many class="ActorRole"/>
</list>

            HasMany(x => x.Contracts)
                .AsSet().Access.CamelCaseField()
                .KeyColumn("ТакаяТоOrganizationId").Inverse().Cascade.SaveUpdate().Where("DeletionDate is null");

      <set access="field.camelcase" cascade="save-update" inverse="true" name="Contracts" where="DeletionDate is null" mutable="true">
        <key foreign-key="FK_ТакаяТоOrganization_Contract_ТакаяТоOrganizationId">
          <column name="ТакаяТоOrganizationId" />
        </key>
        <one-to-many class="..." />
      </set>