Amending Existing JSON Files in Powershell 4

Question

I have seen many questions around this however none of the answers seem to work (please correct me if i am wrong)

I have created a function that queries an API and saves the result as a json file.
However I would like to amend the file that is saved

powershell code:

$search = ""


function searchMtgApi($searchName){
    $uriSafeName = [uri]::EscapeDataString($searchName)
    $res = Invoke-WebRequest "https://api.magicthegathering.io/v1/cards?name=$uriSafeName" -UseBasicParsing
    $resJson = $res.content | ConvertFrom-Json
    $resJson.cards | Format-Table
    $resJson.cards | Select name, setName, "quantity" | ConvertTo-Json | Out-File "D:\Magic the Gathering\$searchName.json"
}

while ($search -ne "exit"){
    Write-Host @'

To exit, type "Exit".

'@
    $search = Read-Host "Search Magic the Gathering.IO by card name"
    if($search -ne "exit"){
          searchMtgApi $search
    }
}

Json generated (searching for the card 'Ponder')

[
    {
        "name":  "Ponder",
        "setName":  "Commander 2018",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Lorwyn",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Magic 2010",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Magic 2012",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Magic Player Rewards 2008",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Magic Online Promos",
        "quantity":  null
    }
]

What I would like to do is load the file and amend the "quantity" for a particular set.
Can anyone point me in the right direction?

Update -

I will get the json content from a file:

function updateJsonFile($jsonfile){
    $resJson = Get-Content "$jsonfile.json" | ConvertFrom-Json 
    #Edit Quantity here
    $resJson | ConvertFrom-Json | Format-Table
}

As an aside: Only ever use Format-* cmdlets for display formatting; never use them if data must be programmatically processed. Format-* cmdlets output formatting instructions, not data - see this answer. — mklement0
– mklement0, Commented Apr 21, 2019 at 19:29

mklement0 · Accepted Answer · 2019-04-21 19:39:17Z

Focusing just on the core question, with abridged sample input:

The following selectively updates the quantity property for an object with a given setName property value, converting from and to JSON:

$setName = 'Magic 2010' # the set name of interest
$newQuantity = 42       # new quantity

(@'
[
    {
        "name":  "Ponder",
        "setName":  "Commander 2018",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Lorwyn",
        "quantity":  null
    },
    {
        "name":  "Ponder",
        "setName":  "Magic 2010",
        "quantity":  null
    }
]
'@  | ConvertFrom-Json) | 
  ForEach-Object {
    if ($_.setName -eq $setName) {
        $_.quantity = $newQuantity
    }
    $_ # pass the (possibly updated) object through.
  } | ConvertTo-Json

Note the need to enclose the ConvertFrom-Json call in (...), which forces enumeration of the individual objects in the array constructed from the JSON input (see this answer for background information).

The above yields (note the updated last quantity value):

[
  {
    "name": "Ponder",
    "setName": "Commander 2018",
    "quantity": null
  },
  {
    "name": "Ponder",
    "setName": "Lorwyn",
    "quantity": null
  },
  {
    "name": "Ponder",
    "setName": "Magic 2010",
    "quantity": 42
  }
]

Applied to your updateJsonFile function:

function updateJsonFile($jsonfile, $setName, $newQuantity){
    $resJson = Get-Content "$jsonfile.json"
    # Note the (...) around the ConvertFrom-Json command.
    ($resJson | ConvertFrom-Json) | 
      ForEach-Object {
        if ($_.setName -eq $setName) {
          $_.quantity = $newQuantity
        }
        $_ # pass the (possibly updated) object through.
      } | ConvertTo-Json | Set-Content -Encoding Utf8 $jsonfile
}

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