I have 3 objects. Bin, Asset and Tag. Asset has a look up relation with bin_id. Tag has a look up relation with Asset_id. So, I want to have an action item on Bin where people can scan tag_id which can look up asset_id and update bin_id in Asset table.

Hi,

try this approach this will help you.

 To achieve the desired functionality, you'll need to create a custom action in Salesforce that allows users to scan the tag_id, look up the corresponding asset_id, and then update the bin_id in the Asset object. Here are the steps to implement this solution: 

Step 1: Create a Visualforce Page or Lightning Component

<apex:page controller="TagScanController">
    <apex:form>
        <apex:pageBlock title="Scan Tag">
            <apex:pageBlockSection>
                <apex:inputText value="{!tagId}" label="Tag ID" />
                <apex:commandButton value="Submit" action="{!updateBinId}" />
            </apex:pageBlockSection>
        </apex:pageBlock>
    </apex:form>
</apex:page>

Step 2: Create the Apex Controller

public class TagScanController {
    public String tagId { get; set; }
    public void updateBinId() {
        if (String.isNotBlank(tagId)) {
            // Lookup the asset_id using tag_id
            Tracking_Tag__c tag = [SELECT Asset_Id__c FROM Tracking_Tag__c WHERE Tag_Id__c = :tagId LIMIT 1];
            if (tag != null) {
                // Get the current Bin ID from the Asset
                Bin_Assets__c asset = [SELECT Bin_Id__c FROM Bin_Assets__c WHERE Id = :tag.Asset_Id__c LIMIT 1];
                if (asset != null) {
                    // Update the Bin ID in the Asset
                    asset.Bin_Id__c = 'NEW_BIN_ID'; // Replace 'NEW_BIN_ID' with the desired Bin ID
                    update asset;
                    ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.INFO, 'Bin ID updated successfully.'));
                } else {
                    ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Asset not found.'));
                }
            } else {
                ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Tag not found.'));
            }
        } else {
            ApexPages.addMessage(new ApexPages.Message(ApexPages.Severity.ERROR, 'Tag ID cannot be blank.'));
        }
    }
}

Step 3: Add the Action to the Bin Record Page

Add the Visualforce page or Lightning component to the Bin record page layout to make it easily accessible.

Thanks

Bablu Pandit