<p>In <a href="https://github.com/dbsrgits/dbix-class/pull/88#discussion_r43756701">lib/DBIx/Class/ResultSource.pm</a>:</p>
<pre style='color:#555'>&gt; +=item Return Value: Hashref of unique constraint data
&gt; +
&gt; +=back
&gt; +
&gt; +  my $unique_info = $source-&gt;unique_constraints_info();
&gt; +
&gt; +Read-only accessor returning all information about unique constraints.
&gt; +
&gt; +The hashref is keyed by the constraint name, and the values are
&gt; +hashrefs contaning two keys:
&gt; +
&gt; +C&lt;columns&gt; is an arrayref of the columns originally provided to
&gt; +L&lt;/add_unique_constraint&gt; or L&lt;/add_unique_constraints&gt;;
&gt; +
&gt; +C&lt;extra&gt; is the hashref originally passed to
&gt; +L&lt;/add_unique_constraint&gt;, or an empty hashref if none was provided.
</pre>
<p>Do not do <code>an empty hashref if none was provided</code>. Either the user supplied one or they did not, do not add anything on your own.</p>

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