Implement $elemMatch

Author: erikdubbelboer

filter/converter.go

@@ -19,6 +19,11 @@ var BasicOperatorMap = map[string]string{
 	"$regex": "~*",
 }
 
+// ReservedColumnName is a reserved column name used internally for nested $elemMatch queries.
+// This column name should not be used in the database or any JSONB column.
+// You can set this to a different value as long as it's a valid Postgres identifier.
+var ReservedColumnName = "__placeholder"
+
 type Converter struct {
 	nestedColumn     string
 	nestedExemptions []string
@@ -150,6 +155,35 @@ func (c *Converter) convertFilter(filter map[string]any, paramIndex int) (string
 							v[operator] = c.arrayDriver(v[operator])
 						}
 						values = append(values, v[operator])
+					case "$elemMatch":
+						// $elemMatch needs a different implementation depending on if the column is in JSONB or not.
+						isNestedColumn := c.nestedColumn != ""
+						for _, exemption := range c.nestedExemptions {
+							if exemption == key {
+								isNestedColumn = false
+								break
+							}
+						}
+						innerConditions, innerValues, err := c.convertFilter(map[string]any{ReservedColumnName: v[operator]}, paramIndex)
+						if err != nil {
+							return "", nil, err
+						}
+						paramIndex += len(innerValues)
+						if isNestedColumn {
+							// This will for example become:
+							//
+							//   EXISTS (SELECT 1 FROM jsonb_array_elements("meta"->'foo') AS __placeholder WHERE ("__placeholder"::text = $1))
+							//
+							// We can't use c.columnName here because we need `->` to get the jsonb value instead of `->>` which gets the text value.
+							inner = append(inner, fmt.Sprintf("EXISTS (SELECT 1 FROM jsonb_array_elements(%q->'%s') AS %s WHERE %s)", c.nestedColumn, key, ReservedColumnName, innerConditions))
+						} else {
+							// This will for example become:
+							//
+							//   EXISTS (SELECT 1 FROM unnest("foo") AS __placeholder WHERE ("__placeholder"::text = $1))
+							//
+							inner = append(inner, fmt.Sprintf("EXISTS (SELECT 1 FROM unnest(%s) AS %s WHERE %s)", c.columnName(key), ReservedColumnName, innerConditions))
+						}
+						values = append(values, innerValues...)
 					default:
 						value := v[operator]
 						op, ok := BasicOperatorMap[operator]
@@ -182,6 +216,9 @@ func (c *Converter) convertFilter(filter map[string]any, paramIndex int) (string
 }
 
 func (c *Converter) columnName(column string) string {
+	if column == ReservedColumnName {
+		return fmt.Sprintf(`%q::text`, column)
+	}
 	if c.nestedColumn == "" {
 		return fmt.Sprintf("%q", column)
 	}

filter/converter_test.go

@@ -208,6 +208,30 @@ func TestConverter_Convert(t *testing.T) {
 			nil,
 			fmt.Errorf("empty objects not allowed"),
 		},
+		{
+			"$elemMatch on normal column",
+			nil,
+			`{"name": {"$elemMatch": {"$eq": "John"}}}`,
+			`EXISTS (SELECT 1 FROM unnest("name") AS __placeholder WHERE ("__placeholder"::text = $1))`,
+			[]any{"John"},
+			nil,
+		},
+		{
+			"$elemMatch on jsonb column",
+			filter.WithNestedJSONB("meta"),
+			`{"name": {"$elemMatch": {"$eq": "John"}}}`,
+			`EXISTS (SELECT 1 FROM jsonb_array_elements("meta"->'name') AS __placeholder WHERE ("__placeholder"::text = $1))`,
+			[]any{"John"},
+			nil,
+		},
+		{
+			"$elemMatch with $gt",
+			nil,
+			`{"age": {"$elemMatch": {"$gt": 18}}}`,
+			`EXISTS (SELECT 1 FROM unnest("age") AS __placeholder WHERE ("__placeholder"::text > $1))`,
+			[]any{float64(18)},
+			nil,
+		},
 	}
 
 	for _, tt := range tests {

fuzz/fuzz_test.go

@@ -33,6 +33,8 @@ func FuzzConverter(f *testing.F) {
 		`{"name": {}}`,
 		`{"$or": []}`,
 		`{"status": {"$in": []}}`,
+		`{"name": {"$elemMatch": {"$eq": "John"}}}`,
+		`{"age": {"$elemMatch": {"$gt": 18}}}`,
 	}
 	for _, tc := range tcs {
 		f.Add(tc)

integration/postgres_test.go

@@ -297,6 +297,30 @@ func TestIntegration_BasicOperators(t *testing.T) {
 			[]int{},
 			nil,
 		},
+		{
+			"$elemMatch on normal column",
+			`{"items": {"$elemMatch": {"$regex": "a"}}}`,
+			[]int{5, 6},
+			nil,
+		},
+		{
+			"$elemMatch on jsonb column",
+			`{"hats": {"$elemMatch": {"$regex": "a"}}}`,
+			[]int{6},
+			nil,
+		},
+		{
+			"$elemMatch with a numeric column",
+			`{"parents": {"$elemMatch": {"$gt": 40, "$lt": 60}}}`,
+			[]int{3},
+			nil,
+		},
+		{
+			"$elemMatch with numeric jsonb column",
+			`{"keys": {"$elemMatch": {"$gt": 5}}}`,
+			[]int{3},
+			nil,
+		},
 	}
 
 	for _, tt := range tests {