Add GQL stuff that I accidentally removed ._.

docs/src/designing-a-schema/types.md

@@ -2,6 +2,122 @@
 
 ## Objects
 
+Object types are the most commonly used type in indexer GraphQL schema. Each object type marked with an `@entity` directive will be converted into a SQL table. 
+
+```graphql
+type Account @entity {
+    id: ID!
+    address: Address!
+    balance: UInt8!
+}
+```
+
+This `Account` object type from the GraphQL schema, might be used in an indexer module like so:
+
+```rust, ignore
+extern crate alloc;
+use fuel_indexer_utils::prelude::*;
+#[indexer(manifest = "indexer.manifest.yaml")]
+mod indexer_mod {
+    fn handle_event(event: Event) {
+        let address = Address::default();
+        let balance = 0;
+        let account = Account::new(address, balance);
+        account.save();
+    }
+}
+```
+
 ## Enums
 
-## Unions
+Enum types are simply implemented as String types.
+
+```graphql
+enum SignatureLabel {
+    Multi
+    Single
+}
+```
+
+> Enum types in relation to Fuel indexer's implementation are just `String` types used primarily to label object types. There is no other way that `enum` types should be used at this time.
+This `SignatureLabel` object type from the GraphQL schema, might be used in an indexer module like so:
+
+```rust, ignore
+extern crate alloc;
+use fuel_indexer_utils::prelude::*;
+#[indexer(manifest = "indexer.manifest.yaml")]
+mod indexer_mod {
+    fn handle_event(event: Event) {
+        let label = SignatureLabel::Multi;
+        assert_eq!(label.to_string(), "SignatureLabel::Multi".to_string());
+    }
+}
+```
+
+## Unions
+
+Union types are unique in that any type marked as a `union` will be converted into an Object type, who's fields are the unique set of fields over all members of the union. 
+
+```graphql
+enum TransactionLabel {
+    Create
+    Script
+    Mint
+}
+
+type CreateTransaction @entity {
+    id: ID!
+    bytecode_length: UInt8!
+    contract_id: ContractId!
+    label: TransactionLabel!
+}
+
+type ScriptTransaction @entity {
+    id: ID!
+    maturity: UInt8!
+    label: TransactionLabel!
+}
+
+type MintTransaction @entity {
+    id: ID!
+    metadata: Json
+    label: TransactionLabel!
+}
+
+union Transaction = CreateTransaction | ScriptTransaction | MintTransaction
+```
+
+The `Transaction` union type above, will internally produce the following object type:
+
+```graphql
+type Transaction @entity {
+    id: ID!
+    bytecode_length: UInt8!
+    contract_id: ContractId!
+    label: TransactionLabel!
+    maturity: UInt8!
+    metadata: Json
+}
+```
+
+> IMPORTANT: Note the order of the fields in the derived `Transaction` object type: the fields are ordered according to the unique set of fields from each of the union's members.
+>
+> The `id`, `bytecode_length`, `contract_id`, and `label` fields come first, from the `CreateTransaction` object type. Next comes the `maturity` field from the `ScriptTransaction` object - because the `ScriptTransaction`'s `id` and `label` fiels are already a part of the derived `Transaction` object, courtesy of the `CreateTransaction` object type. Finally, comes the `metadata` field, as part of the `MintTransaction` object type.
+This `Transaction` union type from the GraphQL schema, might be used in an indexer module like so:
+
+```rust, ignore
+extern crate alloc;
+use fuel_indexer_utils::prelude::*;
+#[indexer(manifest = "indexer.manifest.yaml")]
+mod indexer_mod {
+    fn handle_event(event: Event) {
+        let bytecode_length = 1024;
+        let contract_id = ContractId::default();
+        let label = TransactionLabel::Create;
+        let maturity = 10000000;
+        let metadata = None;
+        let transaction = Transaction::new(bytecode_length, contract_id, label, maturity, metadata);
+        transaction.save();
+    }
+}
+```