Support multiple 'alter column' sub operations (#338)

Allow 'alter column' operations to include multiple sub-operations. This
means migrations like this one are now possible:

```json
{
  "name": "35_alter_column_multiple",
  "operations": [
    {
      "alter_column": {
        "table": "events",
        "column": "name",
        "name": "event_name",
        "type": "text",
        "nullable": false,
        "unique": {
          "name": "events_event_name_unique"
        },
        "check": {
          "name": "event_name_length",
          "constraint": "length(name) > 3"
        },
        "up": "(SELECT CASE WHEN name IS NULL THEN 'placeholder' ELSE name END)",
        "down": "name"
      }
    }
  ]
}
```

This 'alter column' operation:
* Renames a column
* Changes its type
* Sets it `NOT NULL`
* Adds a unique constraint
* Adds a check constraint

Previously, this would have required 5 different operations.

Builds on #337. Fixes
#336

docs/README.md

@@ -738,6 +738,8 @@ Example **add column** migrations:
 
 An alter column operation alters the properties of a column. The operation supports several sub-operations, described below.
 
+An alter column operation may contain multiple sub-operations. For example, a single alter column operation may rename a column, change its type, and add a check constraint.
+
 #### Rename column
 
 A rename column operation renames a column.

examples/34_create_events_table.json

@@ -0,0 +1,22 @@
+{
+  "name": "34_create_events_table",
+  "operations": [
+    {
+      "create_table": {
+        "name": "events",
+        "columns": [
+          {
+            "name": "id",
+            "type": "serial",
+            "pk": true
+          },
+          {
+            "name": "name",
+            "type": "varchar(255)",
+            "nullable": true
+          }
+        ]
+      }
+    }
+  ]
+}

examples/35_alter_column_multiple.json

@@ -0,0 +1,23 @@
+{
+  "name": "35_alter_column_multiple",
+  "operations": [
+    {
+      "alter_column": {
+        "table": "events",
+        "column": "name",
+        "name": "event_name",
+        "type": "text",
+        "nullable": false,
+        "unique": {
+          "name": "events_event_name_unique"
+        },
+        "check": {
+          "name": "event_name_length",
+          "constraint": "length(name) > 3"
+        },
+        "up": "(SELECT CASE WHEN name IS NULL OR LENGTH(name) <= 3 THEN 'placeholder' ELSE name END)",
+        "down": "name"
+      }
+    }
+  ]
+}

pkg/jsonschema/testdata/alter-column-3.txtar

@@ -1,5 +1,5 @@
 This is an invalid 'alter_column' migration.
-It sets `name` but also sets `up` and `down`.
+It sets only `name` but also sets `up` and `down`.
 
 -- alter_column.json --
 {

pkg/jsonschema/testdata/alter-column-4.txtar

@@ -1,5 +1,5 @@
 This is an invalid 'alter_column' migration.
-It sets `name` but also sets `up`.
+It sets only `name` but also sets `up`.
 
 -- alter_column.json --
 {

pkg/jsonschema/testdata/alter-column-5.txtar

@@ -1,5 +1,5 @@
 This is an invalid 'alter_column' migration.
-It sets `name` but also sets `down`.
+It sets only `name` but also sets `down`.
 
 -- alter_column.json --
 {

pkg/jsonschema/testdata/alter-column-6.txtar

@@ -1,4 +1,4 @@
-This is an invalid 'alter_column' migration.
+This is a valid 'alter_column' migration.
 It sets both `name` and `nullable`.
 
 -- alter_column.json --
@@ -19,4 +19,4 @@ It sets both `name` and `nullable`.
 }
 
 -- valid --
-false
+true

pkg/jsonschema/testdata/alter-column-7.txtar

@@ -1,5 +1,5 @@
-This is an invalid 'alter_column' migration.
-It sets both `name` and `nullable`.
+This is a invalid 'alter_column' migration.
+It sets both `name` and `nullable` but does not set `up` or `down`.
 
 -- alter_column.json --
 {

pkg/jsonschema/testdata/alter-column-8.txtar

@@ -1,4 +1,4 @@
-This is an invalid 'alter_column' migration.
+This is a valid 'alter_column' migration.
 It sets both `type` and `nullable`.
 
 -- alter_column.json --
@@ -19,4 +19,4 @@ It sets both `type` and `nullable`.
 }
 
 -- valid --
-false
+true

pkg/jsonschema/testdata/alter-column-9.txtar

@@ -0,0 +1,18 @@
+This is an invalid 'alter_column' migration.
+It specifies a table and column but no changes.
+
+-- alter_column.json --
+{
+  "name": "migration_name",
+  "operations": [
+    {
+      "alter_column": {
+        "table": "reviews",
+        "column": "review"
+      }
+    }
+  ]
+}
+
+-- valid --
+false

pkg/migrations/errors.go

@@ -158,14 +158,6 @@ func (e NoDownSQLAllowedError) Error() string {
 	return "down SQL is not allowed for this operation"
 }
 
-type MultipleAlterColumnChangesError struct {
-	Changes int
-}
-
-func (e MultipleAlterColumnChangesError) Error() string {
-	return fmt.Sprintf("alter column operations require exactly one change, found %d", e.Changes)
-}
-
 type BackfillNotPossibleError struct {
 	Table string
 }
@@ -199,3 +191,12 @@ func (e InvalidOnDeleteSettingError) Error() string {
 		e.Setting,
 	)
 }
+
+type AlterColumnNoChangesError struct {
+	Table  string
+	Column string
+}
+
+func (e AlterColumnNoChangesError) Error() string {
+	return fmt.Sprintf("alter column %q on table %q requires at least one change", e.Column, e.Table)
+}