Add child schema format to parent schema when serializing as V2

Author: MaggieKimani1

src/Microsoft.OpenApi/Models/OpenApiSchema.cs

@@ -707,6 +707,12 @@ internal void WriteAsSchemaProperties(
                     // oneOf (Not Supported in V2) - Write the first schema only as an allOf.
                     writer.WriteOptionalCollection(OpenApiConstants.AllOf, OneOf?.Take(1), (w, s) => s.SerializeAsV2(w));
                 }
+                if (OneOf?.Count > 0)
+                {
+                    // Take the format and set it at the root
+                    var oneOfFormat = OneOf.Select<OpenApiSchema, string>(x => x.Format.ToString()).FirstOrDefault();
+                    this.Format = oneOfFormat;
+                }
             }
 
             // properties

test/Microsoft.OpenApi.Tests/Models/OpenApiSchemaTests.cs

@@ -422,5 +422,68 @@ public async Task SerializeSchemaWRequiredPropertiesAsV2JsonWorksAsync(bool prod
             // Assert
             await Verifier.Verify(actual).UseParameters(produceTerseOutput);
         }
+
+        [Fact]
+        public void SerializeSchemaPrimitiveTypeShouldRemoveFormatInRootIfPresentInChildrenSchema()
+        {
+            // Arrange
+            var schema = new OpenApiSchema()
+            {
+                OneOf = new List<OpenApiSchema>
+                {
+                    new OpenApiSchema
+                    { 
+                        Type = "number",
+                        Format = "decimal"
+                    },
+                    new OpenApiSchema { Type = "string" },
+                }
+            };
+
+            var outputStringWriter = new StringWriter(CultureInfo.InvariantCulture);
+            var openApiJsonWriter = new OpenApiJsonWriter(outputStringWriter, new OpenApiJsonWriterSettings { Terse = false });
+
+            // Act
+            // Serialize as V2
+            schema.SerializeAsV2(openApiJsonWriter);
+            openApiJsonWriter.Flush();
+
+            var v2Schema = outputStringWriter.GetStringBuilder().ToString();//.Replace(Environment.NewLine, "").Replace(" ", "").Replace("\n","");
+
+            // Serialize as V3
+            //schema.SerializeAsV3(openApiJsonWriter);
+            //openApiJsonWriter.Flush();
+
+            //var v3Schema = outputStringWriter.GetStringBuilder().ToString();//.Replace(Environment.NewLine, "").Replace(" ", "").Replace("\n", "");
+
+            var expectedV2Schema = @"{
+""allOf"": [
+{
+    ""format"": ""decimal"",
+    ""type"": ""number""
+}],
+""format"": ""decimal""
+}".Replace(Environment.NewLine, "").Replace(" ", "").Replace("\n","");
+
+            
+            var expectedV3Schema = @"{
+""allOf"": [
+{
+    ""format"": ""decimal"",
+    ""type"": ""number""
+}]}
+{""oneOf"": [
+{
+    ""type"": ""number"",
+    ""format"": ""decimal""
+},
+{""type"" : ""string""}
+
+]}}";//.Replace(Environment.NewLine, "").Replace(" ", "").Replace("\n", "");
+
+            // Assert
+            Assert.Equal(expectedV2Schema, v2Schema); // Assert that v2 schema has the root schema Format defined
+            //Assert.Equal(expectedV3Schema, v3Schema);
+        }
     }
 }