Add $field operator to compare fields (#23)

Currently you can only compare fields to constants. This new operator
allows you to compare fields with fields.

For example `playerCount < maxPlayers`

This is an exception to the mongodb syntax as mongodb doesn't support
this without `$expr` which we don't support because it's not JSON
compatible.

---------

Co-authored-by: Koen Bollen <meneer@koenbollen.nl>

Author: erikdubbelboer

README.md

@@ -7,9 +7,10 @@ _It's designed to be simple, secure, and free of dependencies._
 When filtering data based on user-generated inputs, you need a syntax that's both intuitive and reliable. MongoDB's query filter is an excellent choice because it's simple, widely understood, and battle-tested in real-world applications. Although this package doesn't interact with MongoDB, it uses the same syntax to simplify filtering.
 
 ### Supported Features:
-- Basics: `$eq`, `$ne`, `$gt`, `$gte`, `$lt`, `$lte`, `$regex`
-- Logical operators: `$and`, `$or`
-- Array operators: `$in`
+- Basics: `$eq`, `$ne`, `$gt`, `$gte`, `$lt`, `$lte`, `$regex`, `$exists`
+- Logical operators: `$and`, `$or`, `$not`, `$nor`
+- Array operators: `$in`, `$nin`, `$elemMatch`
+- Field comparison: `$field` (see [#difference-with-mongodb](#difference-with-mongodb))
 
 This package is intended for use with PostgreSQL drivers like [github.com/lib/pq](https://github.com/lib/pq) and [github.com/jackc/pgx](https://github.com/jackc/pgx). However, it can work with any driver that supports the database/sql package.
 
@@ -92,6 +93,19 @@ values := []any{"aztec", "nuke", "", 2, 10}
 (given "customdata" is configured with `filter.WithNestedJSONB("customdata", "password", "playerCount")`)
 
 
+## Difference with MongoDB
+
+- The MongoDB query filters don't have the option to compare fields with each other. This package adds the `$field` operator to compare fields with each other.  
+For example:
+```json5
+{
+  "playerCount": { "$lt": { "$field": "maxPlayers" } }
+}
+```
+
+- Some comparisons have limitations.`>`, `>=`, `<` and `<=` only work on non-jsob fields if they are numeric.
+
+
 ## Contributing
 
 If you have a feature request or discovered a bug, we'd love to hear from you! Please open an issue or submit a pull request. This project adheres to the [Poki Vulnerability Disclosure Policy](https://poki.com/en/c/vulnerability-disclosure-policy).

filter/converter.go

@@ -235,6 +235,13 @@ func (c *Converter) convertFilter(filter map[string]any, paramIndex int) (string
 							inner = append(inner, fmt.Sprintf("EXISTS (SELECT 1 FROM unnest(%s) AS %s WHERE %s)", c.columnName(key), c.placeholderName, innerConditions))
 						}
 						values = append(values, innerValues...)
+					case "$field":
+						vv, ok := v[operator].(string)
+						if !ok {
+							return "", nil, fmt.Errorf("invalid value for $field operator (must be string): %v", v[operator])
+						}
+
+						inner = append(inner, fmt.Sprintf("(%s = %s)", c.columnName(key), c.columnName(vv)))
 					default:
 						value := v[operator]
 						isNumericOperator := false
@@ -247,19 +254,41 @@ func (c *Converter) convertFilter(filter map[string]any, paramIndex int) (string
 							isNumericOperator = true
 						}
 
-						// Prevent cryptic errors like:
-						// 	 unexpected error: sql: converting argument $1 type: unsupported type []interface {}, a slice of interface
-						if !isScalar(value) {
-							return "", nil, fmt.Errorf("invalid comparison value (must be a primitive): %v", value)
-						}
+						// If the value is a map with a $field key, we need to compare the column to another column.
+						if vv, ok := value.(map[string]any); ok {
+							field, ok := vv["$field"].(string)
+							if !ok || len(vv) > 1 {
+								return "", nil, fmt.Errorf("invalid value for %s operator (must be object with $field key only): %v", operator, value)
+							}
+
+							left := c.columnName(key)
+							right := c.columnName(field)
 
-						if isNumericOperator && isNumeric(value) && c.isNestedColumn(key) {
-							inner = append(inner, fmt.Sprintf("((%s)::numeric %s $%d)", c.columnName(key), op, paramIndex))
+							if isNumericOperator {
+								if c.isNestedColumn(key) {
+									left = fmt.Sprintf("(%s)::numeric", left)
+								}
+								if c.isNestedColumn(field) {
+									right = fmt.Sprintf("(%s)::numeric", right)
+								}
+							}
+
+							inner = append(inner, fmt.Sprintf("(%s %s %s)", left, op, right))
 						} else {
-							inner = append(inner, fmt.Sprintf("(%s %s $%d)", c.columnName(key), op, paramIndex))
+							// Prevent cryptic errors like:
+							// 	 unexpected error: sql: converting argument $1 type: unsupported type []interface {}, a slice of interface
+							if !isScalar(value) {
+								return "", nil, fmt.Errorf("invalid comparison value (must be a primitive): %v", value)
+							}
+
+							if isNumericOperator && isNumeric(value) && c.isNestedColumn(key) {
+								inner = append(inner, fmt.Sprintf("((%s)::numeric %s $%d)", c.columnName(key), op, paramIndex))
+							} else {
+								inner = append(inner, fmt.Sprintf("(%s %s $%d)", c.columnName(key), op, paramIndex))
+							}
+							paramIndex++
+							values = append(values, value)
 						}
-						paramIndex++
-						values = append(values, value)
 					}
 				}
 				innerResult := strings.Join(inner, " AND ")

filter/converter_test.go

@@ -382,6 +382,46 @@ func TestConverter_Convert(t *testing.T) {
 			nil,
 			fmt.Errorf("invalid comparison value (must be a primitive): [1 2]"),
 		},
+		{
+			"compare two fields",
+			nil,
+			`{"playerCount": {"$lt": {"$field": "maxPlayers"}}}`,
+			`("playerCount" < "maxPlayers")`,
+			nil,
+			nil,
+		},
+		{
+			"compare two jsonb fields",
+			filter.WithNestedJSONB("meta"),
+			`{"foo": {"$eq": {"$field": "bar"}}}`,
+			`("meta"->>'foo' = "meta"->>'bar')`,
+			nil,
+			nil,
+		},
+		{
+			"compare two jsonb fields with numeric comparison",
+			filter.WithNestedJSONB("meta"),
+			`{"foo": {"$lt": {"$field": "bar"}}}`,
+			`(("meta"->>'foo')::numeric < ("meta"->>'bar')::numeric)`,
+			nil,
+			nil,
+		},
+		{
+			"compare two fields with simple expression",
+			filter.WithNestedJSONB("meta", "foo"),
+			`{"foo": {"$field": "bar"}}`,
+			`("foo" = "meta"->>'bar')`,
+			nil,
+			nil,
+		},
+		{
+			"compare with invalid object",
+			nil,
+			`{"name": {"$eq": {"foo": "bar"}}}`,
+			``,
+			nil,
+			fmt.Errorf("invalid value for $eq operator (must be object with $field key only): map[foo:bar]"),
+		},
 	}
 
 	for _, tt := range tests {

integration/postgres_test.go

@@ -399,6 +399,48 @@ func TestIntegration_BasicOperators(t *testing.T) {
 			[]int{},
 			nil,
 		},
+		{
+			"string order comparison",
+			`{"pet": {"$lt": "dog"}}`,
+			[]int{2, 4, 6, 8},
+			nil,
+		},
+		{
+			"compare two fields",
+			`{"level": {"$lt": { "$field": "guild_id" }}}`,
+			[]int{1},
+			nil,
+		},
+		{
+			"compare two string fields",
+			`{"name": {"$field": "pet"}}`,
+			[]int{},
+			nil,
+		},
+		{
+			"compare two string fields with jsonb",
+			`{"pet": {"$field": "class"}}`,
+			[]int{3},
+			nil,
+		},
+		{
+			// This converts to: ("level" = "metadata"->>'guild_id')
+			// This currently doesn't work, because we don't know the type of the columns.
+			// 'level' is an integer column, 'guild_id' is a jsonb column which always gets converted to a string.
+			"compare two numeric fields",
+			`{"level": {"$field": "guild_id"}}`,
+			nil,
+			errors.New(`pq: operator does not exist: integer = text`),
+		},
+		{
+			// This converts to: (("metadata"->>'pet')::numeric < "class")
+			// This currently doesn't work, because we always convert < etc to a numeric comparison.
+			// We don't know the type of the columns, so we can't convert it to a string comparison.
+			"string order comparison with two fields",
+			`{"pet": {"$lt": {"$field": "class"}}}`,
+			nil,
+			errors.New(`pq: operator does not exist: numeric < text`),
+		},
 	}
 
 	for _, tt := range tests {

integration/setup_test.go

@@ -127,7 +127,7 @@ func createPlayersTable(t *testing.T, db *sql.DB) {
 			("id", "name",    "metadata",                                           "level", "class",    "mount",   "items",              "parents") VALUES
 			(1,    'Alice',   '{"guild_id": 20, "pet": "dog"                    }', 10,      'warrior',  'horse',   '{}',                 '{40, 60}'),
 			(2,    'Bob',	  '{"guild_id": 20, "pet": "cat", "keys": [1, 3]    }', 20,      'mage',     'horse',   '{}',                 '{20, 30}'),
-			(3,    'Charlie', '{"guild_id": 30, "pet": "dog", "keys": [4, 6]    }', 30,      'rogue',    NULL,      '{}',                 '{30, 50}'),
+			(3,    'Charlie', '{"guild_id": 30, "pet": "dog", "keys": [4, 6]    }', 30,      'dog',      NULL,      '{}',                 '{30, 50}'),
 			(4,    'David',   '{"guild_id": 30, "pet": "cat"                    }', 40,      'warrior',  NULL,      '{}',                 '{}'),
 			(5,    'Eve',     '{"guild_id": 40, "pet": "dog", "hats": ["helmet"]}', 50,      'mage',     'griffon', '{"staff", "cloak"}', '{}'),
 			(6,    'Frank',   '{"guild_id": 40, "pet": "cat", "hats": ["cap"]   }', 60,      'rogue',    'griffon', '{"dagger"}',         '{}'),