add instance model

apps/explorer/lib/explorer/chain/token/instance.ex

@@ -0,0 +1,43 @@
+defmodule Explorer.Chain.Token.Instance do
+  @moduledoc """
+  Represents an ERC 721 token instance and stores metadata defined in https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md.
+  """
+
+  use Explorer.Schema
+
+  alias Explorer.Chain.{Hash, Token}
+  alias Explorer.Chain.Token.Instance
+
+  @typedoc """
+  * `token_id` - ID of the token
+  * `token_contract_address_hash` - Address hash foreign key
+  * `metadata` - Token instance metadata
+  """
+
+  @type t :: %Instance{
+          token_id: non_neg_integer(),
+          token_contract_address_hash: Hash.Address.t(),
+          metadata: Map.t()
+        }
+
+  schema "token_instances" do
+    field(:token_id, :decimal, primary_key: true)
+    field(:metadata, :map)
+
+    belongs_to(
+      :token,
+      Token,
+      foreign_key: :token_contract_address_hash,
+      references: :contract_address_hash,
+      type: Hash.Address,
+      primary_key: true
+    )
+  end
+
+  # def changeset(%Instance{} = instance, params \\ %{}) do
+  #   instance
+  #   |> cast([:token_id, :metadata, :token_contract_address_hash])
+  #   |> validate_required([:token_id, :token_contract_address_hash])
+  #   |> foreign_key_constraint(:token_contract_address_hash)
+  # end
+end

apps/explorer/priv/repo/migrations/20190905083522_create_token_instances.exs

@@ -0,0 +1,18 @@
+defmodule Explorer.Repo.Migrations.CreateTokenInstances do
+  use Ecto.Migration
+
+  def change do
+    create table(:token_instances, primary_key: false) do
+      # ERC-721 tokens have IDs
+      # 10^x = 2^256, x ~ 77.064, so 78 decimal digits will store the full 256-bits of a native EVM type
+      add(:token_id, :numeric, precision: 78, scale: 0, null: false, primary_key: true)
+
+      add(:token_contract_address_hash, references(:tokens, column: :contract_address_hash, type: :bytea),
+        null: false,
+        primary_key: true
+      )
+
+      add(:metadata, :jsonb)
+    end
+  end
+end