Add read-only support for PostgreSQL array columns

PostgreSQL array columns (int[], text[], boolean[], bigint[], etc.) were
previously unsupported and caused initialization failures. This change adds
read-only support so array columns are exposed as list types in GraphQL
([Int], [String], etc.) and as JSON arrays in REST responses.

Array columns are marked read-only and excluded from mutation input types
until write support is implemented. The implementation adds generic array
plumbing in the shared SQL layers and PostgreSQL-specific element type
resolution via information_schema udt_name mapping.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bilby91

src/Config/DatabasePrimitives/DatabaseObject.cs

@@ -282,6 +282,17 @@ public class ColumnDefinition
     public object? DefaultValue { get; set; }
     public int? Length { get; set; }
 
+    /// <summary>
+    /// Indicates whether this column is a database array type (e.g., PostgreSQL int[], text[]).
+    /// </summary>
+    public bool IsArrayType { get; set; }
+
+    /// <summary>
+    /// The CLR type of the array element when <see cref="IsArrayType"/> is true.
+    /// For example, typeof(int) for an int[] column.
+    /// </summary>
+    public Type? ElementSystemType { get; set; }
+
     public ColumnDefinition() { }
 
     public ColumnDefinition(Type systemType)

src/Core/Parsers/EdmModelBuilder.cs

@@ -111,7 +111,8 @@ SourceDefinition sourceDefinition
                     // each column represents a property of the current entity we are adding
                     foreach (string column in sourceDefinition.Columns.Keys)
                     {
-                        Type columnSystemType = sourceDefinition.Columns[column].SystemType;
+                        ColumnDefinition columnDef = sourceDefinition.Columns[column];
+                        Type columnSystemType = columnDef.SystemType;
                         // need to convert our column system type to an Edm type
                         EdmPrimitiveTypeKind type = TypeHelper.GetEdmPrimitiveTypeFromSystemType(columnSystemType);
 
@@ -125,6 +126,14 @@ SourceDefinition sourceDefinition
                             sqlMetadataProvider.TryGetExposedColumnName(entityAndDbObject.Key, column, out exposedColumnName!);
                             newEntity.AddKeys(newEntity.AddStructuralProperty(name: exposedColumnName, type, isNullable: false));
                         }
+                        else if (columnDef.IsArrayType)
+                        {
+                            // Array columns are represented as EDM collection types (e.g., Collection(Edm.Int32) for int[]).
+                            sqlMetadataProvider.TryGetExposedColumnName(entityAndDbObject.Key, column, out exposedColumnName!);
+                            EdmPrimitiveTypeReference elementTypeRef = new(EdmCoreModel.Instance.GetPrimitiveType(type), isNullable: true);
+                            EdmCollectionTypeReference collectionTypeRef = new(new EdmCollectionType(elementTypeRef));
+                            newEntity.AddStructuralProperty(name: exposedColumnName, collectionTypeRef);
+                        }
                         else
                         {
                             sqlMetadataProvider.TryGetExposedColumnName(entityAndDbObject.Key, column, out exposedColumnName!);

src/Core/Services/MetadataProviders/PostgreSqlMetadataProvider.cs

@@ -1,7 +1,9 @@
 // Copyright (c) Microsoft Corporation.
 // Licensed under the MIT License.
 
+using System.Data;
 using System.Net;
+using Azure.DataApiBuilder.Config.DatabasePrimitives;
 using Azure.DataApiBuilder.Core.Configurations;
 using Azure.DataApiBuilder.Core.Resolvers.Factories;
 using Azure.DataApiBuilder.Service.Exceptions;
@@ -75,5 +77,71 @@ public override Type SqlToCLRType(string sqlType)
         {
             throw new NotImplementedException();
         }
+
+        /// <summary>
+        /// Maps PostgreSQL array udt_name prefixes to their CLR element types.
+        /// PostgreSQL array types in information_schema use udt_name with a leading underscore
+        /// (e.g., _int4 for int[], _text for text[]).
+        /// </summary>
+        private static readonly Dictionary<string, Type> _pgArrayUdtToElementType = new(StringComparer.OrdinalIgnoreCase)
+        {
+            ["_int2"] = typeof(short),
+            ["_int4"] = typeof(int),
+            ["_int8"] = typeof(long),
+            ["_float4"] = typeof(float),
+            ["_float8"] = typeof(double),
+            ["_numeric"] = typeof(decimal),
+            ["_bool"] = typeof(bool),
+            ["_text"] = typeof(string),
+            ["_varchar"] = typeof(string),
+            ["_bpchar"] = typeof(string),
+            ["_uuid"] = typeof(Guid),
+            ["_timestamp"] = typeof(DateTime),
+            ["_timestamptz"] = typeof(DateTimeOffset),
+        };
+
+        /// <summary>
+        /// Override to detect PostgreSQL array columns using information_schema metadata.
+        /// Npgsql's DataAdapter reports array columns as System.Array (the abstract base class),
+        /// so we use the data_type and udt_name from information_schema.columns to identify arrays
+        /// and resolve their element types.
+        /// </summary>
+        protected override void PopulateColumnDefinitionWithHasDefaultAndDbType(
+            SourceDefinition sourceDefinition,
+            DataTable allColumnsInTable)
+        {
+            foreach (DataRow columnInfo in allColumnsInTable.Rows)
+            {
+                string columnName = (string)columnInfo["COLUMN_NAME"];
+                bool hasDefault =
+                    Type.GetTypeCode(columnInfo["COLUMN_DEFAULT"].GetType()) != TypeCode.DBNull;
+
+                if (sourceDefinition.Columns.TryGetValue(columnName, out ColumnDefinition? columnDefinition))
+                {
+                    columnDefinition.HasDefault = hasDefault;
+
+                    if (hasDefault)
+                    {
+                        columnDefinition.DefaultValue = columnInfo["COLUMN_DEFAULT"];
+                    }
+
+                    // Detect array columns: data_type is "ARRAY" in information_schema for PostgreSQL array types.
+                    string dataType = columnInfo["DATA_TYPE"] is string dt ? dt : string.Empty;
+                    if (string.Equals(dataType, "ARRAY", StringComparison.OrdinalIgnoreCase))
+                    {
+                        string udtName = columnInfo["UDT_NAME"] is string udt ? udt : string.Empty;
+                        if (_pgArrayUdtToElementType.TryGetValue(udtName, out Type? elementType))
+                        {
+                            columnDefinition.IsArrayType = true;
+                            columnDefinition.ElementSystemType = elementType;
+                            columnDefinition.SystemType = elementType.MakeArrayType();
+                            columnDefinition.IsReadOnly = true;
+                        }
+                    }
+
+                    columnDefinition.DbType = TypeHelper.GetDbTypeFromSystemType(columnDefinition.SystemType);
+                }
+            }
+        }
     }
 }

src/Core/Services/MetadataProviders/SqlMetadataProvider.cs

@@ -1479,14 +1479,24 @@ private async Task PopulateSourceDefinitionAsync(
                        subStatusCode: DataApiBuilderException.SubStatusCodes.ErrorInInitialization);
                 }
 
+                Type systemType = (Type)columnInfoFromAdapter["DataType"];
+
+                // Detect array types: concrete array types (e.g., int[]) have IsArray=true,
+                // while Npgsql reports abstract System.Array for PostgreSQL array columns.
+                // byte[] is excluded since it maps to the bytea/ByteArray scalar type.
+                bool isArrayType = (systemType.IsArray && systemType != typeof(byte[])) || systemType == typeof(Array);
+
                 ColumnDefinition column = new()
                 {
                     IsNullable = (bool)columnInfoFromAdapter["AllowDBNull"],
                     IsAutoGenerated = (bool)columnInfoFromAdapter["IsAutoIncrement"],
-                    SystemType = (Type)columnInfoFromAdapter["DataType"],
+                    SystemType = systemType,
+                    IsArrayType = isArrayType,
+                    ElementSystemType = isArrayType && systemType.IsArray ? systemType.GetElementType() : null,
                     // An auto-increment column is also considered as a read-only column. For other types of read-only columns,
                     // the flag is populated later via PopulateColumnDefinitionsWithReadOnlyFlag() method.
-                    IsReadOnly = (bool)columnInfoFromAdapter["IsAutoIncrement"]
+                    // Array columns are also treated as read-only until write support for array types is implemented.
+                    IsReadOnly = (bool)columnInfoFromAdapter["IsAutoIncrement"] || isArrayType
                 };
 
                 // Tests may try to add the same column simultaneously

src/Core/Services/TypeHelper.cs

@@ -135,6 +135,12 @@ public static EdmPrimitiveTypeKind GetEdmPrimitiveTypeFromSystemType(Type column
             {
                 columnSystemType = columnSystemType.GetElementType()!;
             }
+            else if (columnSystemType == typeof(Array))
+            {
+                // Npgsql may report abstract System.Array for unresolved PostgreSQL array columns.
+                // Default to String if the element type hasn't been resolved yet.
+                return EdmPrimitiveTypeKind.String;
+            }
 
             EdmPrimitiveTypeKind type = columnSystemType.Name switch
             {

src/Service.GraphQLBuilder/Sql/SchemaConverter.cs

@@ -441,7 +441,13 @@ private static FieldDefinitionNode GenerateFieldForColumn(Entity configEntity, s
                 }
             }
 
-            NamedTypeNode fieldType = new(GetGraphQLTypeFromSystemType(column.SystemType));
+            NamedTypeNode namedType = new(GetGraphQLTypeFromSystemType(column.SystemType));
+
+            // For array columns, wrap the element type in a ListTypeNode (e.g., [Int], [String]).
+            INullableTypeNode fieldType = column.IsArrayType
+                ? new ListTypeNode(namedType)
+                : namedType;
+
             FieldDefinitionNode field = new(
                 location: null,
                 new(exposedColumnName),
@@ -541,6 +547,19 @@ private static List<DirectiveNode> GenerateObjectTypeDirectivesForEntity(string
         /// GraphQL type.</exception>"
         public static string GetGraphQLTypeFromSystemType(Type type)
         {
+            // For array types (e.g., int[], string[]), resolve the element type.
+            // byte[] is excluded as it maps to the ByteArray scalar type.
+            if (type.IsArray && type != typeof(byte[]))
+            {
+                type = type.GetElementType()!;
+            }
+            else if (type == typeof(Array))
+            {
+                // Npgsql may report abstract System.Array for unresolved PostgreSQL array columns.
+                // Default to String if the element type hasn't been resolved yet.
+                return STRING_TYPE;
+            }
+
             return type.Name switch
             {
                 "String" => STRING_TYPE,

src/Service.Tests/DatabaseSchema-PostgreSql.sql

@@ -22,6 +22,7 @@ DROP TABLE IF EXISTS stocks_price;
 DROP TABLE IF EXISTS stocks;
 DROP TABLE IF EXISTS comics;
 DROP TABLE IF EXISTS brokers;
+DROP TABLE IF EXISTS array_type_table;
 DROP TABLE IF EXISTS type_table;
 DROP TABLE IF EXISTS trees;
 DROP TABLE IF EXISTS fungi;
@@ -166,6 +167,14 @@ CREATE TABLE type_table(
     uuid_types uuid DEFAULT gen_random_uuid ()
 );
 
+CREATE TABLE array_type_table(
+    id int GENERATED BY DEFAULT AS IDENTITY PRIMARY KEY,
+    int_array_col int[],
+    text_array_col text[],
+    bool_array_col boolean[],
+    long_array_col bigint[]
+);
+
 CREATE TABLE trees (
     "treeId" int PRIMARY KEY,
     species text,
@@ -412,6 +421,10 @@ INSERT INTO type_table(id, short_types, int_types, long_types, string_types, sin
     (4, 32767, 2147483647, 9223372036854775807, 'null', 3.4E38, 1.7E308, 2.929292E-14, true, '9999-12-31 23:59:59.997', '\xFFFFFFFF'),
     (5, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL);
 INSERT INTO type_table(id, uuid_types) values(10, 'D1D021A8-47B4-4AE4-B718-98E89C41A161');
+INSERT INTO array_type_table(id, int_array_col, text_array_col, bool_array_col, long_array_col) VALUES
+    (1, '{1,2,3}', '{hello,world}', '{true,false}', '{100,200,300}'),
+    (2, '{10,20}', '{foo,bar,baz}', '{true,true}', '{999}'),
+    (3, NULL, NULL, NULL, NULL);
 INSERT INTO trees("treeId", species, region, height) VALUES (1, 'Tsuga terophylla', 'Pacific Northwest', '30m'), (2, 'Pseudotsuga menziesii', 'Pacific Northwest', '40m');
 INSERT INTO trees("treeId", species, region, height) VALUES (4, 'test', 'Pacific Northwest', '0m');
 INSERT INTO fungi(speciesid, region, habitat) VALUES (1, 'northeast', 'forest'), (2, 'southwest', 'sand');