Implement $exists (#17)

See:
https://www.mongodb.com/docs/manual/reference/operator/query/exists/

Author: erikdubbelboer

filter/converter.go

@@ -174,6 +174,24 @@ func (c *Converter) convertFilter(filter map[string]any, paramIndex int) (string
 							v[operator] = c.arrayDriver(v[operator])
 						}
 						values = append(values, v[operator])
+					case "$exists":
+						// $exists only works on jsonb columns, so we need to check if the key is in the JSONB data first.
+						isNestedColumn := c.nestedColumn != ""
+						for _, exemption := range c.nestedExemptions {
+							if exemption == key {
+								isNestedColumn = false
+								break
+							}
+						}
+						if !isNestedColumn {
+							// There is no way in Postgres to check if a column exists on a table.
+							return "", nil, fmt.Errorf("$exists operator not supported on non-nested jsonb columns")
+						}
+						neg := ""
+						if v[operator] == false {
+							neg = "NOT "
+						}
+						inner = append(inner, fmt.Sprintf("(%sjsonb_path_match(%s, 'exists($.%s)'))", neg, c.nestedColumn, key))
 					default:
 						value := v[operator]
 						op, ok := basicOperatorMap[operator]

filter/converter_test.go

@@ -248,14 +248,6 @@ func TestConverter_Convert(t *testing.T) {
 			nil,
 			fmt.Errorf("invalid value for $not operator (must be object): John"),
 		},
-		{
-			"sql injection",
-			nil,
-			`{"\"bla = 1 --": 1}`,
-			``,
-			nil,
-			fmt.Errorf("invalid column name: \"bla = 1 --"),
-		},
 		{
 			"compare with array",
 			nil,
@@ -264,14 +256,6 @@ func TestConverter_Convert(t *testing.T) {
 			nil,
 			fmt.Errorf("invalid comparison value (must be a primitive): [200 300]"),
 		},
-		{
-			"sql injection",
-			nil,
-			`{"\"bla = 1 --": 1}`,
-			``,
-			nil,
-			fmt.Errorf("invalid column name: \"bla = 1 --"),
-		},
 		{
 			"null nornal column",
 			nil,
@@ -288,6 +272,30 @@ func TestConverter_Convert(t *testing.T) {
 			nil,
 			nil,
 		},
+		{
+			"$exists on normal column",
+			nil,
+			`{"name": {"$exists": false}}`,
+			``,
+			nil,
+			fmt.Errorf("$exists operator not supported on non-nested jsonb columns"),
+		},
+		{
+			"not $exists jsonb column",
+			filter.WithNestedJSONB("meta"),
+			`{"name": {"$exists": false}}`,
+			`(NOT jsonb_path_match(meta, 'exists($.name)'))`,
+			nil,
+			nil,
+		},
+		{
+			"$exists jsonb column",
+			filter.WithNestedJSONB("meta"),
+			`{"name": {"$exists": true}}`,
+			`(jsonb_path_match(meta, 'exists($.name)'))`,
+			nil,
+			nil,
+		},
 	}
 
 	for _, tt := range tests {

fuzz/fuzz_test.go

@@ -40,6 +40,7 @@ func FuzzConverter(f *testing.F) {
 		`{"$not": {"name": "John"}}`,
 		`{"name": {"$not": {"$eq": "John"}}}`,
 		`{"name": null}`,
+		`{"name": {"$exists": false}}`,
 	}
 	for _, tc := range tcs {
 		f.Add(tc, true)

integration/postgres_test.go

@@ -333,6 +333,18 @@ func TestIntegration_BasicOperators(t *testing.T) {
 			[]int{10},
 			nil,
 		},
+		{
+			`jsonb exists`,
+			`{"pet": {"$exists": false}}`,
+			[]int{9},
+			nil,
+		},
+		{
+			`jsonb exists`,
+			`{"pet": {"$exists": true}}`,
+			[]int{1, 2, 3, 4, 5, 6, 7, 8, 10},
+			nil,
+		},
 	}
 
 	for _, tt := range tests {