title: How do I delete the key by knowing its values in flutter?
body: <p>I have got a map of objects:</p>
  
  <pre class="lang-dart prettyprint-override"><code>Map&lt;String, Transaction&gt; _userIncome = {
      'four': Transaction(amount: 450, date: DateTime.now(), title: 'Einkommen', accountType: 'timr', notes: 'joa' , icon: Icon(Icons.today,), id: 'kololdcd', repeat: 'always'),
      'five': Transaction(amount: 60, date: DateTime.now(), title: 'Bruder', accountType: 'timr', notes: 'brother' , icon: Icon(Icons.today,), id: 'kolkfrcd', repeat: 'never'),
      'six': Transaction(amount: 60, date: DateTime.now(), title: 'Rückerstattung', accountType: 'timr', notes: 'brother' , icon: Icon(Icons.today,), id: 'kolofkfrcd', repeat: 'never'),
  };
  </code></pre>
  
  <p>And I would like to get the key if I know the values (the values are unique), so that I can delete the the entry by its key: </p>
  
  <pre class="lang-dart prettyprint-override"><code>_userIncome.remove(key);
  </code></pre>
  
  <p>I can identify the entry with the given values: </p>
  
  <pre class="lang-dart prettyprint-override"><code>_userIncome.values
      .where((t) =&gt; t.date.difference(datte).inDays == 0 &amp;&amp; t.id == transactionId);
  </code></pre>
  
  <p>But I have no clue how I get the matching key, so that I can delete the entry. Or is there a way to the delete an entry by its values?</p>
  
  <p>I would love to hear some advice :)</p>