TEMP: Add investigation dir

Add some example migrations to reproduce the problem.

investigation/01_create_tables.json

@@ -0,0 +1,42 @@
+{
+  "name": "01_create_tables",
+  "operations": [
+    {
+      "create_table": {
+        "name": "employees",
+        "columns": [
+          {
+            "name": "id",
+            "type": "serial",
+            "pk": true
+          },
+          {
+            "name": "name",
+            "type": "varchar(255)"
+          },
+          {
+            "name": "department_id",
+            "type": "integer",
+            "nullable": true
+          }
+        ]
+      }
+    },
+    {
+      "create_table": {
+        "name": "departments",
+        "columns": [
+          {
+            "name": "id",
+            "type": "serial",
+            "pk": true
+          },
+          {
+            "name": "name",
+            "type": "varchar(255)"
+          }
+        ]
+      }
+    }
+  ]
+}

investigation/02_add_fk.json

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+{
+  "name": "02_add_fk",
+  "operations": [
+    {
+      "alter_column": {
+        "table": "employees",
+        "column": "department_id",
+        "references": {
+          "name": "fk_employees_department_id",
+          "table": "departments",
+          "column": "id"
+        },
+        "up": "''",
+        "down": "''"
+      }
+    }
+  ]
+}

investigation/03_set_fk_not_null.json

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+{
+  "name": "03_set_fk_not_null",
+  "operations": [
+    {
+      "alter_column": {
+        "table": "employees",
+        "column": "department_id",
+        "nullable": false,
+        "up": "(SELECT CASE WHEN department_id IS NULL THEN 1 ELSE department_id END)",
+        "down": ""
+      }
+    }
+  ]
+}

investigation/insert.sql

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+insert into departments (name) values ('engineering'), ('marketing');
+insert into employees (name, department_id) values ('alice', 1), ('bob', 2);