how to upload files using Apex code

ContentDocument object does not allow insert DML operation in Salesforce, so we can upload it through the ContentVersion object, without ContentDocumentId. After DML on ContentVersion a new version of ContentDocument will be created for us in salesforce.

The easiest way to do this is using inputFile and assign it to ContentVersion instance. Like this:

<apex:page controller="ContentController">
<apex:form>
    <apex:inputFile value="{!file}" />
    <apex:commandbutton action="{!upload}" value="Upload" />
</apex:form>
</apex:page>

Class:

public class ContentController {
    public blob file { get; set; }

    public PageReference upload() {
        ContentVersion v = new ContentVersion();
        v.versionData = file;
        v.title = 'testing upload';
        v.pathOnClient ='/somepath.txt';
        insert v;
        return new PageReference('/' + v.id);
    }
}

If you want to share the ContentVersion file then after insert DML query the ContentDocumentId from inserted ContentVersion and use ContentDocumentLink to create association between Record and ContentVersion uploaded file.

https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_contentdocumentlink.htm

Use "LinkedEntityId" field to place the record id to which this file will be associated.

Note: LinkedEntityId - Can include Chatter users, groups, records (any that support Chatter feed tracking including custom objects), and Salesforce CRM Content libraries.

hope this helps and mark this as answer if does. Thanks

I found a way to insert ContentVersion and attach it to Record in one SOQL.

<apex:page controller="ContentController">
    <apex:form>
        <apex:inputFile value="{!file}" />
        <apex:commandbutton action="{!upload}" value="Upload" />
    </apex:form>
</apex:page>

Class:

public class ContentController {
    public blob file { get; set; }

    public PageReference upload() {
        ContentVersion v = new ContentVersion();
        v.versionData = file;
        v.title = 'testing upload';
        v.pathOnClient ='/somepath.txt';
        v.FirstPublishLocationId = parentObj.Id; //similar to parentid
        insert v;
        return new PageReference('/' + v.id);
    }
}