This works similarly to https://gist.github.com/fulv/7260167. The only difference is that form1 and form2 have a flditems field, which is a datagrid, and uses formdatagrid as its associated form. Formdatagrid has two fields, a text fldname, and a number fldquantity. After creating a document with form1, containing a few datagrid rows, open form2.

gistfile1.txt

<plominodatabase id="testdb2">
  <design>
    <dbsettings version="1.18">
      <AboutDescription type="Products.Archetypes.Field.TextField"><![CDATA[]]></AboutDescription>
      <UsingDescription type="Products.Archetypes.Field.TextField"><![CDATA[]]></UsingDescription>
      <IndexAttachments type="Products.Archetypes.Field.BooleanField">False</IndexAttachments>
      <FulltextIndex type="Products.Archetypes.Field.BooleanField">True</FulltextIndex>
      <IndexInPortal type="Products.Archetypes.Field.BooleanField">False</IndexInPortal>
      <debugMode type="Products.Archetypes.Field.BooleanField">False</debugMode>
      <CountDocuments type="Products.Archetypes.Field.BooleanField">False</CountDocuments>
      <DateTimeFormat type="Products.Archetypes.Field.StringField">%Y-%m-%d</DateTimeFormat>
      <StartPage type="Products.Archetypes.Field.StringField"/>
      <i18n type="Products.Archetypes.Field.StringField"/>
      <DoNotListUsers type="Products.Archetypes.Field.BooleanField">False</DoNotListUsers>
      <DoNotReindex type="Products.Archetypes.Field.BooleanField">False</DoNotReindex>
      <IsDatabaseTemplate type="Products.Archetypes.Field.BooleanField">False</IsDatabaseTemplate>
      <excludeFromNav type="Products.Archetypes.Field.BooleanField">False</excludeFromNav>
      <acl AnomynousAccessRight="NoAccess" AuthenticatedAccessRight="NoAccess">
        <params id="UserRoles">
<param>
<value><struct>
</struct></value>
</param>
</params>
        <params id="SpecificRights">
<param>
<value><struct>
<member>
<name>specific_deletedocument</name>
<value><string>PlominoAuthor</string></value>
</member>
</struct></value>
</param>
</params>
      </acl>
    </dbsettings>
    <element id="form1" title="form1" type="PlominoForm">
      <id type="Products.Archetypes.Field.StringField">form1</id>
      <onCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onCreateDocument>
      <onOpenDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onOpenDocument>
      <beforeSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></beforeSaveDocument>
      <onSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSaveDocument>
      <onDeleteDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onDeleteDocument>
      <onSearch type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSearch>
      <beforeCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></beforeCreateDocument>
      <FormLayout type="Products.Archetypes.Field.TextField"><![CDATA[
<p><span class="plominoFieldClass">flditems</span></p>
<p><span class="plominoFieldClass">fldtext</span></p>
]]></FormLayout>
      <FormMethod type="Products.Archetypes.Field.TextField"><![CDATA[Auto]]></FormMethod>
      <DocumentTitle type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentTitle>
      <DynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</DynamicDocumentTitle>
      <StoreDynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</StoreDynamicDocumentTitle>
      <DocumentId type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentId>
      <ActionBarPosition type="Products.Archetypes.Field.StringField">TOP</ActionBarPosition>
      <HideDefaultActions type="Products.Archetypes.Field.BooleanField">False</HideDefaultActions>
      <HideInMenu type="Products.Archetypes.Field.BooleanField">False</HideInMenu>
      <isSearchForm type="Products.Archetypes.Field.BooleanField">False</isSearchForm>
      <isPage type="Products.Archetypes.Field.BooleanField">False</isPage>
      <SearchView type="Products.Archetypes.Field.StringField"/>
      <SearchFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></SearchFormula>
      <Position type="Products.Archetypes.Field.IntegerField"/>
      <ResourcesJS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesJS>
      <ResourcesCSS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesCSS>
      <excludeFromNav type="Products.Archetypes.Field.BooleanField">False</excludeFromNav>
      <elements>
        <element id="flditems" title="flditems" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">flditems</id>
          <FieldType type="Products.Archetypes.Field.StringField">DATAGRID</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
          <params>
<param>
<value><struct>
<member>
<name>associated_form</name>
<value><string>formdatagrid</string></value>
</member>
<member>
<name>jssettings</name>
<value><string>"aoColumns": [
    { "sTitle": "Name" },
    { "sTitle": "Quantity", "sClass": "center" }
],
"bPaginate": false,
"bLengthChange": false,
"bFilter": false,
"bSort": false,
"bInfo": false,
"bAutoWidth": false,
"plominoDialogOptions": {
        "width": 400,
        "height": 300
    }
</string></value>
</member>
<member>
<name>field_mapping</name>
<value><string>fldname,fldquantity</string></value>
</member>
</struct></value>
</param>
</params>
        </element>
        <element id="fldtext" title="fldtext" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">fldtext</id>
          <FieldType type="Products.Archetypes.Field.StringField">TEXT</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
        </element>
      </elements>
    </element>
    <element id="form2" title="form2" type="PlominoForm">
      <id type="Products.Archetypes.Field.StringField">form2</id>
      <onCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onCreateDocument>
      <onOpenDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onOpenDocument>
      <beforeSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></beforeSaveDocument>
      <onSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSaveDocument>
      <onDeleteDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onDeleteDocument>
      <onSearch type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSearch>
      <beforeCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[db = context.getParentDatabase()
search = db.getIndex().dbsearch({})
olddoc = search[0].getObject()
fields = context.getFormFields()
fieldsdict = {f.id:f for f in fields}
for id in fieldsdict:
    value = olddoc.getItem(id)
    Log("id: %s, value: %s" % (id, value))
    context.REQUEST.set(id, value)

return None]]></beforeCreateDocument>
      <FormLayout type="Products.Archetypes.Field.TextField"><![CDATA[
<p><span class="plominoFieldClass">flditems</span></p>
<p><span class="plominoFieldClass">fldtext</span></p>
]]></FormLayout>
      <FormMethod type="Products.Archetypes.Field.TextField"><![CDATA[Auto]]></FormMethod>
      <DocumentTitle type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentTitle>
      <DynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</DynamicDocumentTitle>
      <StoreDynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</StoreDynamicDocumentTitle>
      <DocumentId type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentId>
      <ActionBarPosition type="Products.Archetypes.Field.StringField">TOP</ActionBarPosition>
      <HideDefaultActions type="Products.Archetypes.Field.BooleanField">False</HideDefaultActions>
      <HideInMenu type="Products.Archetypes.Field.BooleanField">False</HideInMenu>
      <isSearchForm type="Products.Archetypes.Field.BooleanField">False</isSearchForm>
      <isPage type="Products.Archetypes.Field.BooleanField">False</isPage>
      <SearchView type="Products.Archetypes.Field.StringField"/>
      <SearchFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></SearchFormula>
      <Position type="Products.Archetypes.Field.IntegerField"/>
      <ResourcesJS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesJS>
      <ResourcesCSS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesCSS>
      <excludeFromNav type="Products.Archetypes.Field.BooleanField">False</excludeFromNav>
      <elements>
        <element id="flditems" title="flditems" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">flditems</id>
          <FieldType type="Products.Archetypes.Field.StringField">DATAGRID</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
          <params>
<param>
<value><struct>
<member>
<name>associated_form</name>
<value><string>formdatagrid</string></value>
</member>
<member>
<name>jssettings</name>
<value><string>"aoColumns": [
    { "sTitle": "Name" },
    { "sTitle": "Quantity", "sClass": "center" }
],
"bPaginate": false,
"bLengthChange": false,
"bFilter": false,
"bSort": false,
"bInfo": false,
"bAutoWidth": false,
"plominoDialogOptions": {
        "width": 400,
        "height": 300
    }
</string></value>
</member>
<member>
<name>field_mapping</name>
<value><string>fldname,fldquantity</string></value>
</member>
</struct></value>
</param>
</params>
        </element>
        <element id="fldtext" title="fldtext" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">fldtext</id>
          <FieldType type="Products.Archetypes.Field.StringField">TEXT</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
        </element>
      </elements>
    </element>
    <element id="formdatagrid" title="formdatagrid" type="PlominoForm">
      <id type="Products.Archetypes.Field.StringField">formdatagrid</id>
      <onCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onCreateDocument>
      <onOpenDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onOpenDocument>
      <beforeSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></beforeSaveDocument>
      <onSaveDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSaveDocument>
      <onDeleteDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></onDeleteDocument>
      <onSearch type="Products.Archetypes.Field.TextField"><![CDATA[]]></onSearch>
      <beforeCreateDocument type="Products.Archetypes.Field.TextField"><![CDATA[]]></beforeCreateDocument>
      <FormLayout type="Products.Archetypes.Field.TextField"><![CDATA[
<p><span class="plominoFieldClass">fldname</span></p>
<p><span class="plominoFieldClass">fldquantity</span></p>
]]></FormLayout>
      <FormMethod type="Products.Archetypes.Field.TextField"><![CDATA[Auto]]></FormMethod>
      <DocumentTitle type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentTitle>
      <DynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</DynamicDocumentTitle>
      <StoreDynamicDocumentTitle type="Products.Archetypes.Field.BooleanField">False</StoreDynamicDocumentTitle>
      <DocumentId type="Products.Archetypes.Field.TextField"><![CDATA[]]></DocumentId>
      <ActionBarPosition type="Products.Archetypes.Field.StringField">TOP</ActionBarPosition>
      <HideDefaultActions type="Products.Archetypes.Field.BooleanField">False</HideDefaultActions>
      <HideInMenu type="Products.Archetypes.Field.BooleanField">False</HideInMenu>
      <isSearchForm type="Products.Archetypes.Field.BooleanField">False</isSearchForm>
      <isPage type="Products.Archetypes.Field.BooleanField">False</isPage>
      <SearchView type="Products.Archetypes.Field.StringField"/>
      <SearchFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></SearchFormula>
      <Position type="Products.Archetypes.Field.IntegerField"/>
      <ResourcesJS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesJS>
      <ResourcesCSS type="Products.Archetypes.Field.TextField"><![CDATA[]]></ResourcesCSS>
      <excludeFromNav type="Products.Archetypes.Field.BooleanField">False</excludeFromNav>
      <elements>
        <element id="fldname" title="fldname" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">fldname</id>
          <FieldType type="Products.Archetypes.Field.StringField">TEXT</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
        </element>
        <element id="fldquantity" title="fldquantity" type="PlominoField">
          <id type="Products.Archetypes.Field.StringField">fldquantity</id>
          <FieldType type="Products.Archetypes.Field.StringField">NUMBER</FieldType>
          <FieldMode type="Products.Archetypes.Field.StringField">EDITABLE</FieldMode>
          <Formula type="Products.Archetypes.Field.TextField"><![CDATA[]]></Formula>
          <FieldReadTemplate type="Products.Archetypes.Field.StringField"/>
          <FieldEditTemplate type="Products.Archetypes.Field.StringField"/>
          <Mandatory type="Products.Archetypes.Field.BooleanField">False</Mandatory>
          <ValidationFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></ValidationFormula>
          <ToBeIndexed type="Products.Archetypes.Field.BooleanField">False</ToBeIndexed>
          <IndexType type="Products.Archetypes.Field.StringField">DEFAULT</IndexType>
          <HTMLAttributesFormula type="Products.Archetypes.Field.TextField"><![CDATA[]]></HTMLAttributesFormula>
        </element>
      </elements>
    </element>
  </design>
</plominodatabase>