What happens when data is updated in thinkphp

$result = $User->where ("u_id=". $user _id)->setfield (' Img_url ', $url);
if ($result!== false) {
Echo ' Data updated successfully! ';
}else{
Echo ' hasn't updated any data! ';
}

When the value of the hash is not changed, when multiple commits are made,
The first success, the back is false.

What kind of mechanism is this? You know, Aries explain.

Reply to discussion (solution)

Look at the manual said that you also need to pass a condition can be saved successfully, if the u_id is the primary key.
Modify the code as follows: $result = $User->where ("u_id=". $user _id)->setfield (Array (' u_id ', ' Img_url '), Array ($user _id, $url));

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