@CheckResult @NonNull
  public final <T> Observable<T> mapToOne(@NonNull Function<Cursor, T> mapper) {
    return lift(Query.mapToOne(mapper));
  }
@CheckResult @NonNull
  public final <T> Observable<T> mapToOne(@NonNull Function<Cursor, T> mapper) {
    return lift(Query.mapToOne(mapper));
  }
@ CheckResult @ NonNull public final < T > Observable < T > mapToOne ( @ NonNull Function < Cursor , T > mapper ) { return lift ( Query . mapToOne ( mapper ) ) ; }
Given a function mapping the current row of a {@link Cursor} to {@code T}, transform each
emitted {@link Query} which returns a single row to {@code T}.
<p>
It is an error for a query to pass through this operator with more than 1 row in its result
set. Use {@code LIMIT 1} on the underlying SQL query to prevent this. Result sets with 0 rows
do not emit an item.
<p>
This method is equivalent to:
<pre>{@code
flatMap(q -> q.asRows(mapper).take(1))
}</pre>
and a convenience operator for:
<pre>{@code
lift(Query.mapToOne(mapper))
}</pre>

@param mapper Maps the current {@link Cursor} row to {@code T}. May not return null.
Given a function mapping the current row of a { @link Cursor } to { @code T } transform each emitted { @link Query } which returns a single row to { @code T } . <p > It is an error for a query to pass through this operator with more than 1 row in its result set . Use { @code LIMIT 1 } on the underlying SQL query to prevent this . Result sets with 0 rows do not emit an item . <p > This method is equivalent to : <pre > { @code flatMap ( q - > q . asRows ( mapper ) . take ( 1 )) } < / pre > and a convenience operator for : <pre > { @code lift ( Query . mapToOne ( mapper )) } < / pre >