Foreign key constraint sql error from entries/saveEntry when adding entry

Question

I am getting a foreign key constraint sql error when trying to add an entry through a front end form.

<form method="post" accept-charset="UTF-8">
  {{ getCsrfInput() }}
  <input type="hidden" name="action" value="entries/saveEntry">
  <input type="hidden" name="entryId" value="{{ entry.id }}">
  <select name="fields[vendor][]">
    {% for v in craft.entries.section('vendors') %}
      <option value="v.id">{{v.title}}</option>
    {% endfor %}
  </select>
  <input type="submit" value="Submit"/>
</form>

vendor is an Entries fieldtype, for unlimited entries from the vendors channel. Very similar code for adding a category works fine.

The error (complete with back-ticks):

CDbCommand failed to execute the SQL statement: SQLSTATE[23000]: Integrity constraint violation: 1452 Cannot add or update a child row: a foreign key constraint fails (metro.craft_relations, CONSTRAINT craft_relations_targetId_fk FOREIGN KEY (targetId) REFERENCES craft_elements (id) ON DELETE CASCADE). The SQL statement executed was: INSERT INTO craft_relations (fieldId, sourceId, sourceLocale, targetId, sortOrder, dateCreated, dateUpdated, uid) VALUES (:row0_col0, :row0_col1, NULL, :row0_col3, :row0_col4, :row0_col5, :row0_col6, :row0_col7). Bound with :row0_col0='13', :row0_col1='39', :row0_col3='v.id', :row0_col4=1, :row0_col5='2016-02-13 08:28:03', :row0_col6='2016-02-13 08:28:03', :row0_col7='e455a06e-8f9d-4299-ad57-93577907f9a0'

Answer 1

I think the problem is this:

<option value="v.id">{{v.title}}</option>

Try changing it to:

<option value="{{ v.id }}">{{v.title}}</option>