make cid docs generic to any record dwyl/mvp#410

src/mvp/15-item-cid.md

@@ -1,21 +1,27 @@
-# Use `cid` for `item` _unique_ `id`
+# Use `cid` for _universally unique_ `ids`
 
-By default the `items` table uses an auto-incrementing (`Serial`) 
-for the `id` (Primary Key):
+By default all the tables 
+in a `Phoenix` Application 
+use an auto-incrementing (`Serial`) 
+for the `id` (Primary Key).
+e.g the `items` table:
 
 ![items-id-integer](https://github.com/dwyl/mvp/assets/194400/d3020e53-2ad8-43ff-b0ed-95251ee21a87)
 
 This is fine for a _server_-side rendered app 
 with a _single_ relational database instance.
+i.e. the server/database controls the id of records.
 But our ambition has always been 
 to build a mobile + offline-first distributed App.
 
 Luckily our friends at 
-[Protocol Labs]()
-(creators of )
+[Protocol Labs](https://github.com/ipfs)
+creators of
 [`IPFS`](https://github.com/dwyl/learn-ipfs)
-have done some groundwork on "Distributed Apps" problem
-and we have a little library that creates `IPFS` compliant
+have done some great groundwork on the 
+["Decentralized Apps"](https://en.wikipedia.org/wiki/Decentralized_application)
+problem so we can build on that.
+We have a little library that creates `IPFS` compliant
 Content Identifiers: 
 [`cid`](https://github.com/dwyl/cid)
 
@@ -26,11 +32,17 @@ item = %{text: "Build PARA System App", person_id: 2, status: 2}
 Cid.cid(item)
 "zb2rhn92tqTt41uFZ3hh3VPnssXjYCW4yDSX7KB39dXZyMtNC"
 ```
+This `cid` string is unique to this content
+therefore creating it on the client (Mobile device)
+will generate the _same_ `cid`
+if the record is created _offline_.
 
 
-## Add `cid` package to `deps`
 
-Add the `cid` package to the `deps` in `mix.exs`:
+
+## Add `excid` package to `deps`
+
+Add the `excid` package to the `deps` in `mix.exs`:
 
 ```elixir
 # Universally Unique Deterministic Content IDs: github.com/dwyl/cid
@@ -55,30 +67,31 @@ when compared to `base32`.
 ```elixir
 "bafkreihght5nbnmn6xbwoakmjyhkiu2naxmwpxgxbp6xkpbiweuetbohde"
 |> String.length()
-49
+59
 ```
 vs.
+
 ```elixir
 "zb2rhn92tqTt41uFZ3hh3VPnssXjYCW4yDSX7KB39dXZyMtNC"
 |> String.length()
-59
+49
 ```
 
-## Create a Migration to add `cid` field to `item`
+## Create `migration`
 
 Using the 
 [`mix ecto.gen.migration`](https://hexdocs.pm/ecto_sql/Mix.Tasks.Ecto.Gen.Migration.html)
 command, 
 create a migration file:
 
 ```sh
-mix ecto.gen.migration add_cid_to_item
+mix ecto.gen.migration add_cid
 ```
 
 You should see output similar to the following:
 
 ```sh
-* creating priv/repo/migrations/20230824153220_add_cid_to_item.exs
+* creating priv/repo/migrations/20230824153220_add_cid.exs
 ```
 
 This tells us that the migration file was created:
@@ -110,7 +123,35 @@ defmodule App.Repo.Migrations.AddCidToItem do
 end
 ```
 
-
 > **Note**: if you're rusty on `migrations`,
 see:
-[devhints.io/phoenix-migrations](https://devhints.io/phoenix-migrations)
+[devhints.io/phoenix-migrations](https://devhints.io/phoenix-migrations)
+
+## Add `cid` field to `item` schema
+
+Open the 
+`lib/app/item.ex`
+file and locate the `schema items do` section.
+Add the line:
+
+```elixir
+field :cid, :string
+```
+
+## Add `:cid` to `changeset/2`
+
+Still in the 
+`lib/app/item.ex`
+file, locate the `changeset/2` function definition
+and change the line: 
+
+```elixir
+|> cast(attrs, [:person_id, :status, :text])
+```
+
+To: 
+
+```elixir
+|> cast(attrs, [:cid, :person_id, :status, :text])
+```
+