String field value length in mongoDB

Question

The data type of the field is String  I would like to fetch the data where character length of field name is greater than 40   I tried these queries but returning error  1    db usercollection find    where    this name length  gt  40      limit 2    output  error          err     TypeError  Cannot read property  length  of undefined near  40           code    16722     this is working in 2 4 9 But my version is 2 6 5

User · Answer

Queries with  where and  expr are slow if there are too many documents   Using  regex is much faster than  where   expr   db usercollection find       name       s S  40         name length  gt   40     or   db usercollection find       name       regex       s S  40           name length  gt   40      This query is the same meaning with  db usercollection find        where    this name  amp  amp  this name length  gt   40       or  db usercollection find         name       exists   true          expr       gte         strLenCP     name     40           I tested each queries for my collection     find  where  10529 359ms  expr  5305 801ms  regex  2516 124ms    count  where  10872 006ms  expr  2630 155ms  regex  158 066ms

User · Answer

This query will give both field value and length   db usercollection aggregate          project             name   1           length      strLenCP    name

User · Answer

For MongoDB 3 6 and newer   The  expr operator allows the use of aggregation expressions within the query language  thus you can leverage the use of  strLenCP operator to check the length of the string as follows   db usercollection find         name       exists   true          expr       gt         strLenCP     name     40             For MongoDB 3 4 and newer   You can also use the aggregation framework with the  redact pipeline operator that allows you to proccess the logical condition with the  cond operator and uses the special operations   KEEP to  keep  the document where the logical condition is true or   PRUNE to  remove  the document where the condition was false   This operation is similar to having a  project pipeline that selects the fields in the collection and creates a new field that holds the result from the logical condition query and then a subsequent  match  except that  redact uses a single pipeline stage which is more efficient   As for the logical condition  there are String Aggregation Operators that you can use  strLenCP operator to check the length of the string  If the length is  gt a specified value  then this is a true match and the document is  kept   Otherwise it is  pruned  and discarded     Consider running the following aggregate operation which demonstrates the above concept   db usercollection aggregate           match      name       exists   true                        redact                   cond                         gt         strLenCP     name     40                        KEEP                      PRUNE                                         limit   2          If using  where  try your query without the enclosing brackets   db usercollection find   where   this name length  gt  40    limit 2     A better query would be to to check for the field s existence and then check the length   db usercollection find  name    type  2    where   this name length  gt  40    limit 2      or    db usercollection find  name    exists  true    where   this name length  gt   40    limit 2      MongoDB evaluates non- where query operations before  where expressions and non- where query statements may use an index  A much better performance is to store the length of the string as another field and then you can index or search on it  applying  where will be much slower compared to that  It s recommended to use JavaScript expressions and the  where operator as a last resort when you can t structure the data in any other way  or when you are dealing with a small subset of data     A different and faster approach that avoids the use of the  where operator is the  regex operator  Consider the following pattern which searches for   db usercollection find   name      type   2    regex       41        limit 2      Note - From the docs      If an index exists for the field  then MongoDB matches the regular   expression against the values in the index  which can be faster than a   collection scan  Further optimization can occur if the regular   expression is a    prefix expression     which means that all potential   matches start with the same string  This allows MongoDB to construct a      range    from that prefix and only match against those values from the   index that fall within that range       A regular expression is a    prefix expression    if it starts with a   caret     or a left anchor   A   followed by a string of simple   symbols  For example  the regex   abc    will be optimized by   matching only against the values from the index that start with abc       Additionally  while   a     a     and   a     match equivalent   strings  they have different performance characteristics  All of these   expressions use an index if an appropriate index exists  however      a     and   a     are slower    a  can stop scanning after   matching the prefix

User · Answer

Here is one of the way in mongodb you can achieve this    db usercollection find    where   this name length  lt  4

User · Answer

I had a similar kind of scenario  but in my case string is not a 1st level attribute  It is inside an object  In here I couldn t find a suitable answer for it  So I thought to share my solution with you all Hope this will help anyone with a similar kind of problem     Parent Collection      Child      name   Random Name    Age  09        Ex  If we need to get only collections that having child s name s length is higher than 10 characters    db getCollection  Parent   find   where  function      for  var field in this Child name         if  this Child name length  gt  10           return true

[mongodb] String field value length in mongoDB

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