links-manipalution

LinkTypes::Holochain, HDK

Links

• Linking from one hash to another, with some optional metadata
• Link components:
  • Base hash: can be public key, entry hash, action hash or external hash (the hash doesn’t exist in this Distributed Hash Table)
  • Target hash: can be of the same types as the base hash
  • Type: app defined link type
  • Tag: app defined arbitrary metadata
• Neither the base nor the target hash need to exist as actions or entries in the DHT
• Defining link types
  • Only in integrity zomes

Link Types

// zomes/integrity/lib.rs
use hdi::prelude::*;
 
// Defining link types
#[hdk_link_types] // Defines all the link types for the zome
enum LinkTypes { // Enum with each entry type as a variant
   AuthorToComment,Must be a Unit variant (without any payload) 
}

Link Actions

create_link()

// zomes/coordinator/lib.rs
use hdk::prelude::*;
use integrity_zome::{LinkTypes, EntryTypes, Comment}; // Import the types defined in our integrity zome
 
#[hdk_extern]
fn create_comment(comment: Comment) -> ExternResult<ActionHash> {
  let comment_action_hash = create_entry(EntryTypes::Comment(comment))?; // Create comment as always
 
  let my_pub_key: AgentPubKey = agent_info()?.agent_initial_pubkey; 
 
  let create_link_action_hash: ActionHash = create_link(
    my_pub_key, // Base hash
    comment_action_hash,  // Target hash
    LinkTypes::AuthorToComment,  // Link Type
    () // Link tag
  )?;
 
  Ok(comment_action_hash)
}

get_links()

• Links are attached to the base hash
  • You can retrieve all the links attached to a given hash as a base

// zomes/coordinator/lib.rs
use hdk::prelude::*;
use integrity_zome::{LinkTypes, EntryTypes, Comment}; // Import the types defined in our integrity zome
 
#[hdk_extern]
fn get_all_comments_by_agent(author: AgentPubKey) -> ExternResult<Vec<Record>> {
  let links: Vec<Link> = get_links(
    author,  // Base hash 
    LinkTypes::AuthorToComment,  // Link Type
    None,  // Filter on link tag prefix
  )?;
 
  let mut comments: Vec<Record> = vec![];
 
  for link in links {
    let maybe_record = get(ActionHash::from(link.target), GetOptions::default())?;
    if let Some(record) = maybe_record {
      comments.push(record);
    }
  }
 
  Ok(comments)
}