Connected.Framework/Connected.Expressions/Evaluation/ExecutionBuilder.cs

using Connected.Entities.Storage;
using Connected.Expressions.Expressions;
using Connected.Expressions.Languages;
using Connected.Expressions.Visitors;
using Connected.Interop;
using System.Data;
using System.Linq.Expressions;

namespace Connected.Expressions.Evaluation;

public sealed class ExecutionBuilder : DatabaseVisitor
{
    private ExecutionBuilder(ExpressionCompilationContext context, Linguist linguist, Expression executor)
    {
        Context = context;
        Linguist = linguist;
        Executor = executor;
    }

    private ExpressionCompilationContext Context { get; }
    private Linguist Linguist { get; }
    private Expression Executor { get; }

    public static Expression<Func<IStorageExecutor, object>> Build(ExpressionCompilationContext context, Linguist linguist, Expression expression, Expression provider)
    {
        var executor = Expression.Parameter(typeof(IStorageExecutor), "executor");
        var builder = new ExecutionBuilder(context, linguist, executor);

        return builder.Build(expression);
    }

    private Expression<Func<IStorageExecutor, object>> Build(Expression expression)
    {
        expression = Visit(expression);
        expression = Expression.Lambda<Func<IStorageExecutor, object>>(expression, (ParameterExpression)Executor);

        return (Expression<Func<IStorageExecutor, object>>)expression;
    }

    protected override Expression VisitProjection(ProjectionExpression projection)
    {
        /*
         * parameterize query
         */
        var commandText = Linguist.Format(projection.Select);
        var command = new StorageOperation
        {
            CommandText = commandText
        };

        foreach (var parameter in Context.Parameters)
        {
            command.AddParameter(new StorageParameter
            {
                Direction = ParameterDirection.Input,
                Name = $"@{parameter.Key}",
                Type = parameter.Value.Type.ToDbType(),
                Value = parameter.Value.Value
            });
        };

        return ExecuteProjection(projection, command);
    }

    private Expression ExecuteProjection(ProjectionExpression projection, IStorageOperation operation)
    {
        var method = nameof(IStorageExecutor.Execute);
        /*
		 * call low-level execute directly on supplied DbQueryProvider
		 */
        Type typeArgument;

        if (projection.Type.IsEnumerable())
            typeArgument = projection.Type.GetGenericArguments()[0];
        else
            typeArgument = projection.Type;

        var constant = Expression.Constant(operation);
        Expression body = Expression.Call(Executor, nameof(IStorageExecutor.Execute), new Type[] { typeArgument }, constant);

        if (projection.Aggregator is not null)// apply aggregator
            body = ExpressionReplacer.Replace(projection.Aggregator.Body, projection.Aggregator.Parameters[0], body);

        return body;
    }
}