Upload file and attach it to contentDocument

Content Document is a document that has been uploaded to a library in Salesforce CRM Content or Salesforce Files. Whenever you insert a ContentVersion, Automatically a version get inserted for the file.The number increments with each version of the document, for example, 1, 2, 3. Use ContentVersion object to create, query, retrieve, search, edit, and update a specific version of a Salesforce CRM Content document or Salesforce file. Use the ContentDocument object to retrieve, query, update, and delete the latest version of a document, but not a content pack, in a library or a Salesforce file. You cannot insert ContentDocument directly, As soon as you insert ContentVersion, a ContentDocument get inserted behind the scene. Supported Calls for ContentDocument

    delete(), describeLayout()describeSObjects(), query(), retrieve(), search(), undelete(), update()

Use ContentDocument object to retrieve, query, update, and delete the latest version of a document, but not a content pack, in a library or a Salesforce file. Use the ContentVersion object to create, query, retrieve, search, edit, and update a specific version of a Salesforce CRM Content document or Salesforce file. for inserting a ContentVersion, You can use below piece of code:-

ContentVersion ContVerFile = new ContentVersion(); ContVerFile.VersionData = Blob.valueOf('string'); ContVerFile.Title = 'title'; ContVerFile.ContentLocation= 's'; ContVerFile.PathOnClient='title.csv'; insert ContVerFile;
In order to link the file with any record, You need to insert Contentdocumentlink with linlkentityid equals to the id of the record.
Use Contentdocumentlink object to query the locations where a file is shared or query which files are linked to a particular location. For example, the following query returns a particular document shared with a Chatter group:
SELECT ContentDocument.title FROM ContentDocumentLink WHERE ContentDocumentId = '069D00000000so2' AND LinkedEntityId = '0D5000000089123'
Insertion:-
Id conDoc = [SELECT ContentDocumentId FROM ContentVersion WHERE Id =:ContVerFile.Id].ContentDocumentId; ContentDocumentLink cDe = new ContentDocumentLink(); cDe.ContentDocumentId = conDoc; cDe.LinkedEntityId = parentId; cDe.ShareType = 'I'; cDe.Visibility = 'AllUsers'; insert cDe;

You can read more about the Special Access Rules, fields definition and its usage here:-

• ContentVersion
  
• ContentDocument
  
• ContentDocumentLink