MongoDB - How to Update Array fields as part of Documents in MongoDB using Robo 3T - II

On continue from previous post lets see more options to update array.

$push - Adds an item to an array

If you want to add elements without checking (elements exist or not) use $push instead of $addtoset. $push will add the given object to field whether it exists or not.

Example - 1 - Lets check how $push modifier works.

Below command I executed 2 times. And see the behaviour of $push.

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:["red","brown"]}}    } )

Lets check the output. You can see elements “red” and “brown” added two times.

$position: The $position modifier specifies the location in the array at which the $push operator inserts elements. Without the $position modifier, the $push operator inserts elements to the end of the array

Syntax: {   $push: {     <field>: {        $each: [ <value1>, <value2>, ... ],        $position: <num>     }   } }

Example - 2: Add two elements at position 0, means at the beginning. By default / without $position elements will add in last.

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:["white1","white2"],                         $position:0                       }             }    } )

Similarly $position = 2 means new elements will add after color(2). $position = -1 then new elemnets will add before color(5)

$slice  : - The $slice modifier limits the number of array elements during a $push operation. To project, or return, a specified number of array elements from a read operation, see the $slice projection operator instead.

Syntax: {   $push: {      <field>: {        $each: [ <value1>, <value2>, ... ],        $slice: <num>      }   } }

At initial we have only two elements of color array. In below examples we are trying to add elements with $slice modifier to validate the output.

Example - 3  - Slice from the End of the Array The following operation adds new elements to the color array, and then uses the $slice modifier to trim the array to the last four elements:

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:["white","black","green"],$slice: -4}}    } )

Example - 4  - Slice from the Front of the Array The following operation adds new elements to the scores array, and then uses the $slice modifier to trim the array to the first three elements.

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:["white","black","green"],$slice: 3}}    } )

Example - 5  - Update Array Using Slice Only To update the scores field with just the effects of the $slice modifier, specify the number of elements to slice (e.g. 2) for the $slice modifier and an empty array [] for the $each modifier, as in the following:

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:[],$slice: 2}}    } )

$sort : -  The $sort modifier orders the elements of an array during a $push operation. To use the $sort modifier, it must appear with the $each modifier. You can pass an empty array [] to the $each modifier such that only the $sort modifier has an effect.

Syntax: {   $push: {      <field>: {        $each: [ <value1>, <value2>, ... ],        $sort: <sort specification>      }   } }

db.inventory.update(    { item : "journal" },    {      $push: {"color": {$each:[],$sort: 1}}    } )

$pop: - The $pop operator removes the first or last element of an array. Pass $pop a value of -1 to remove the first element of an array and 1 to remove the last element in an array.

db.inventory.update(    { item : "journal" },    {      $pop: {"color":-1 }    } )

$pull: - The $pull operator removes from an existing array all instances of a value or values that match a specified condition

Initially the color array has 3 elements.

Now we are trying to remove middle element “black” from array using $pull operator.

db.inventory.update(    { item : "canvas" },    {      $pull: {"color":{$in:["black"]} }    } )

You can see color “black” removed from array.

Similarly you can remove elements from multiple array in one command, additionally have to use multi:true