fix bug #531

Author: nikitam

src/EFCore.Ydb/src/Query/Internal/YdbSqlExpressionFactory.cs

@@ -1,13 +1,41 @@
+using System;
+using System.Data;
 using System.Diagnostics.CodeAnalysis;
+using EntityFrameworkCore.Ydb.Storage.Internal.Mapping;
 using Microsoft.EntityFrameworkCore.Query;
 using Microsoft.EntityFrameworkCore.Query.SqlExpressions;
 using Microsoft.EntityFrameworkCore.Storage;
-
 namespace EntityFrameworkCore.Ydb.Query.Internal;
 
 public class YdbSqlExpressionFactory(SqlExpressionFactoryDependencies dependencies) : SqlExpressionFactory(dependencies)
 {
     [return: NotNullIfNotNull("sqlExpression")]
     public override SqlExpression? ApplyTypeMapping(SqlExpression? sqlExpression, RelationalTypeMapping? typeMapping) =>
         base.ApplyTypeMapping(sqlExpression, typeMapping);
+
+    public override SqlExpression Coalesce(SqlExpression left, SqlExpression right,
+        RelationalTypeMapping? typeMapping = null)
+    {
+        // For .Sum(x => x.Decimal) EF generates coalesce(sum(x.Decimal), 0.0)) because SUM must have value
+        var funcExpression = left as SqlFunctionExpression;
+        var constExpression = right as SqlConstantExpression;
+        
+        if (funcExpression != null && constExpression != null && constExpression.TypeMapping != null
+            &&
+            funcExpression.Name.Equals("SUM", StringComparison.OrdinalIgnoreCase)
+            &&
+            constExpression.TypeMapping.DbType == DbType.Decimal
+            &&
+            constExpression.Value != null)
+        {
+            var correctRight = new SqlConstantExpression(constExpression.Value,
+                YdbDecimalTypeMapping.WithMaxPrecision); // in the feature change static max precision/scale to
+                                                         // to dynamically created correct precision/scale
+                                                         // it depends on db scheme and can not correctly define only in code
+            
+            return base.Coalesce(left, correctRight, typeMapping);
+        }
+        
+        return base.Coalesce(left, right, typeMapping);
+    }
 }

src/EFCore.Ydb/src/Storage/Internal/Mapping/YdbDecimalTypeMapping.cs

@@ -7,9 +7,32 @@ public class YdbDecimalTypeMapping : DecimalTypeMapping
 {
     private const byte DefaultPrecision = 22;
     private const byte DefaultScale = 9;
+    
+    private const byte MaxPrecision = 35;
 
     public new static YdbDecimalTypeMapping Default => new();
 
+    static YdbDecimalTypeMapping()
+    {
+        WithMaxPrecision = GetWithMaxPrecision();
+    }
+    
+    public static YdbDecimalTypeMapping WithMaxPrecision { get; }
+
+    private static YdbDecimalTypeMapping GetWithMaxPrecision()
+    {
+        var result = new YdbDecimalTypeMapping(new RelationalTypeMappingParameters(
+                new CoreTypeMappingParameters(
+                    typeof(decimal)),
+                    storeType: "Decimal",
+                    dbType: System.Data.DbType.Decimal,
+                    precision: MaxPrecision,
+                    scale: DefaultScale)
+            );
+        
+        return result;
+    }
+
     public YdbDecimalTypeMapping() : this(
         new RelationalTypeMappingParameters(
             new CoreTypeMappingParameters(typeof(decimal)),
@@ -41,4 +64,9 @@ protected override void ConfigureParameter(DbParameter parameter)
         if (Scale is { } s)
             parameter.Scale = (byte)s;
     }
+
+    protected override string GenerateNonNullSqlLiteral(object value)
+    {
+        return $"Decimal('{base.GenerateNonNullSqlLiteral(value)}', {this.Precision ?? MaxPrecision}, {this.Scale ?? DefaultScale})";
+    }
 }