PropertyValueException when trying to persist a new object

[999cm999]

Hi,

I am getting this persistent error that I can't fix:

> PropertyValueException: not-null property references a null or transient value AstroLab.Research.LearningLab.AstroComet._AstroLab.Research.LearningLab.AstroSystem.CometsBackref

This error happens inside Session.Save. I create a new comet programatically, and I am trying to use Session.Save to persist it to the database.

Googling the error turns up very little info.  

In the database, the two tables look like this:

```

CREATE TABLE cometList (

[systemID] [int] NOT NULL,

[cometID] [int] IDENTITY(1,1) NOT NULL,

[cometName] [nvarchar](255) NOT NULL,

[cometSequence] [int] NOT NULL,

[isLocal] [bit] NULL

);

CREATE TABLE systemList (

[learningID] [int] NOT NULL,

[systemID] [int] IDENTITY(1,1) NOT NULL,

[systemSequence] [int] NOT NULL,

[systemName] [nvarchar](255) NULL,

[systemDateTime] [datetime2](0) NULL,

);

```

In the cometList table, the systemID column is a foreign key(systemList).

Inside my .NetCore Web App, my AstroComet XML mapping looks like this:

```

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"

    schema="dbo" assembly="AstroLab.Learn" namespace="AstroLab.Research.LearningLab">

  <class name="AstroComet" table="cometList">

    <id name="Id" column="cometID">

      <generator class="identity" />

    </id>

    <many-to-one name="System" class="AstroSystem" column="systemID" update="false" insert="false" />

    <property name="Sequence" column="cometSequence" update="false" insert="false" />

    <property name="Title" column="cometName" />

    <property name="IsLocal" column="isLocal"/>

  </class>

</hibernate-mapping>

```

And the AstroSystem mapping looks like this:

```

<?xml version="1.0" encoding="utf-8" ?>

<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"

    schema="dbo" assembly="AstroLab.Learn" namespace="AstroLab.Research.LearningLab">

  <class name="AstroSystem" table="systemList">

    <id name="Id" column="systemID">

      <generator class="identity" />

    </id>

    <many-to-one name="LearningLab" class="LearningLab" column="learningID" insert="false" update="false" />

    <property name="Sequence" column="systemSequence" insert="false" update="false"/>

    <property name="Title" column="systemName" />

    <property name="ResearchDate" column="systemDateTime" />

    <list name="Comets" inverse="false" cascade="all">

      <key column="systemID" not-null="true" />

      <index column="cometSequence" />

      <one-to-many class="AstroComet" />

    </list>

  </class>

</hibernate-mapping>

```

I've searched, tried a few things, rewrote some of the code, but it always gives me this error no matter how I try to fix it.

If anyone has any experience with an error like this, please let me know.

Thanks!