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eocroneocron
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Added paging extension
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‎Eocron.Algorithms.Tests/PagingQueryableTests.cs‎

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using Eocron.Algorithms.Queryable.Paging;
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using FluentAssertions;
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using NUnit.Framework;
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namespace Eocron.Algorithms.Tests
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{
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[TestFixture]
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public class PagingQueryableTests
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{
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private List<TestDbEntity> _items;
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[SetUp]
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public void Setup()
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{
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var now = DateTime.UtcNow;
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_items = new List<TestDbEntity>()
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{
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000001"),Name = "Test1", Modified = now.AddDays(1)},
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000002"),Name = "Test2", Modified = now.AddDays(2)},
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000003"),Name = "Test3", Modified = now.AddDays(3)},
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000004"),Name = "Test4", Modified = now.AddDays(4)},
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000005"),Name = "Test5", Modified = now.AddDays(5)},
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new TestDbEntity(){Id = Guid.Parse("10000000-0000-0000-0000-000000000006"),Name = "Test6", Modified = now.AddDays(6)},
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};
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}
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[Test]
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public void SanityCheck()
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{
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var queryable = _items.AsQueryable();
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var cfg = new PagingConfiguration<TestDbEntity>();
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cfg.AddKeySelector(x=> x.Modified, isDescending: true);
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cfg.AddKeySelector(x=> x.Id);
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var result = new List<TestDbEntity>();
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string ct = null;
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do
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{
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var tmpQuery = queryable.Continue(cfg, ct);
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var tmp = tmpQuery.Take(1).ToList().FirstOrDefault();
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if (tmp != null)
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{
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ct = cfg.GetContinuationToken(tmp);
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result.Add(tmp);
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}
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else
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{
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break;
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}
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} while (ct != null);
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result.Should().Equal(_items.OrderByDescending(x=> x.Modified).ThenBy(x=> x.Id));
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}
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public class TestDbEntity
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{
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public Guid Id { get; set; }
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public string Name { get; set; }
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public DateTime Modified { get; set; }
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}
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}
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}

‎Eocron.Algorithms/Queryable/Paging/PagingConfiguration.cs‎

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using System;
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using System.Collections.Generic;
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using System.Linq;
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using System.Linq.Expressions;
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using Newtonsoft.Json;
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namespace Eocron.Algorithms.Queryable.Paging
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{
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public sealed class PagingConfiguration<TEntity>
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{
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private readonly List<PagingKeyConfiguration> _keys = new();
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public ParameterExpression Input = Expression.Parameter(typeof(TEntity), "x");
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public IReadOnlyList<PagingKeyConfiguration> Keys => _keys;
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private static readonly JsonSerializerSettings JsonSerializerSettings = new JsonSerializerSettings()
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{
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TypeNameHandling = TypeNameHandling.All
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};
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public void AddKeySelector<TKey>(Expression<Func<TEntity, TKey>> keySelector, bool isDescending = false)
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{
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if(keySelector == null)
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throw new ArgumentNullException(nameof(keySelector));
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var compiled = keySelector.Compile();
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keySelector = (Expression<Func<TEntity, TKey>>)new ReplaceParameterVisitor(keySelector.Parameters[0], Input).Visit(keySelector);
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_keys.Add(new PagingKeyConfiguration()
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{
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CompiledKeySelector = x => new TypeWrapper<TKey> { Value = compiled(x) },
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KeySelector = keySelector,
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IsDescending = isDescending
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});
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}
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public string GetContinuationToken(TEntity entity)
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{
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if (entity == null)
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throw new ArgumentNullException(nameof(entity));
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if(_keys.Count == 0)
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throw new InvalidOperationException("No keys defined.");
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return JsonConvert.SerializeObject(_keys.Select(x => x.CompiledKeySelector(entity)).ToList(),
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JsonSerializerSettings);
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}
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public List<object> GetKeyValues(string continuationToken)
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{
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if (string.IsNullOrWhiteSpace(continuationToken))
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throw new ArgumentNullException(nameof(continuationToken));
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if(_keys.Count == 0)
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throw new InvalidOperationException("No keys defined.");
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return JsonConvert.DeserializeObject<List<object>>(continuationToken, JsonSerializerSettings)
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.Cast<ITypeWrapper>()
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.Select(x => x.GetValue())
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.ToList();
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}
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private interface ITypeWrapper
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{
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object GetValue();
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}
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public class PagingKeyConfiguration
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{
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public Func<TEntity, object> CompiledKeySelector { get; init; }
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public LambdaExpression KeySelector { get; init; }
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public bool IsDescending { get; init; }
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}
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private sealed class TypeWrapper<TValue> : ITypeWrapper
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{
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public TValue Value { get; init; }
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public object GetValue()
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{
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return Value;
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}
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}
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private sealed class ReplaceParameterVisitor : ExpressionVisitor
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{
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private readonly ParameterExpression _oldParam;
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private readonly ParameterExpression _newParam;
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public ReplaceParameterVisitor(ParameterExpression oldParam, ParameterExpression newParam)
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{
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_oldParam = oldParam;
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_newParam = newParam;
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}
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protected override Expression VisitParameter(ParameterExpression node)
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{
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return node == _oldParam ? _newParam : base.VisitParameter(node);
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}
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}
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}
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}

‎Eocron.Algorithms/Queryable/Paging/PagingQueryableExtensions.cs‎

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using System;
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using System.Linq;
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using System.Linq.Expressions;
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namespace Eocron.Algorithms.Queryable.Paging
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{
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/// <summary>
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/// Provides extension to efficiently page through any queryable, without full scan
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/// </summary>
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public static class PagingQueryableExtensions
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{
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/// <summary>
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/// Apply WHERE condition to IQueryable if continuation token is not empty.
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/// It will not apply Skip/Take operations, you should do it yourself.
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/// </summary>
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/// <param name="source"></param>
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/// <param name="configuration"></param>
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/// <param name="continuationToken"></param>
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/// <typeparam name="TEntity"></typeparam>
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/// <returns></returns>
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public static IQueryable<TEntity> Continue<TEntity>(
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this IQueryable<TEntity> source,
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PagingConfiguration<TEntity> configuration,
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string continuationToken) where TEntity : class
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{
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if(source == null)
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throw new ArgumentNullException(nameof(source));
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if (configuration == null)
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throw new ArgumentNullException(nameof(configuration));
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var result = source;
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result = ApplyOrdering(result, configuration);
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if (string.IsNullOrWhiteSpace(continuationToken))
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return result;
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result = result.Where(CreateSkipCondition(configuration, continuationToken));
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return result;
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}
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private static Expression<Func<TEntity, bool>> CreateSkipCondition<TEntity>(
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PagingConfiguration<TEntity> configuration,
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string continuationToken)
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{
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var keyValues = configuration.GetKeyValues(continuationToken).Select(Expression.Constant).ToList();
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Expression predicate = null;
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for (var i = configuration.Keys.Count - 1; i >= 0; i--)
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{
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var keyCfg = configuration.Keys[i];
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var keyValue = keyValues[i];
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var comparison = keyCfg.IsDescending
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? Expression.LessThan(keyCfg.KeySelector.Body, keyValue)
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: Expression.GreaterThan(keyCfg.KeySelector.Body, keyValue);
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for (var j = 0; j < i; j++)
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{
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var prevKeyCfg = configuration.Keys[j];
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var prevKeyValue = keyValues[j];
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var prevEqual = Expression.Equal(prevKeyCfg.KeySelector.Body, prevKeyValue);
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comparison = Expression.AndAlso(prevEqual, comparison);
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}
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predicate = predicate == null ? comparison : Expression.OrElse(predicate, comparison);
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}
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var lambda = Expression.Lambda<Func<TEntity, bool>>(predicate!, configuration.Input);
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return lambda;
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}
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private static IQueryable<TEntity> ApplyOrdering<TEntity>(IQueryable<TEntity> queryable,
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PagingConfiguration<TEntity> configuration)
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{
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for (int i = 0; i < configuration.Keys.Count; i++)
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{
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var keyCfg = configuration.Keys[i];
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queryable = ApplyOrdering(queryable, keyCfg.KeySelector, keyCfg.IsDescending, isFirst: i == 0);
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}
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return queryable;
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}
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private static IOrderedQueryable<TEntity> ApplyOrdering<TEntity>(
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IQueryable<TEntity> source,
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LambdaExpression keySelector,
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bool isDescending,
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bool isFirst)
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{
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var methodName = (isFirst, isDescending) switch
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{
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(true, false) => nameof(System.Linq.Queryable.OrderBy),
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(true, true) => nameof(System.Linq.Queryable.OrderByDescending),
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(false, false) => nameof(System.Linq.Queryable.ThenBy),
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(false, true) => nameof(System.Linq.Queryable.ThenByDescending),
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};
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var method = typeof(System.Linq.Queryable)
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.GetMethods()
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.First(m =>
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m.Name == methodName &&
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m.GetParameters().Length == 2)
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.MakeGenericMethod(typeof(TEntity), keySelector.Body.Type);
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return (IOrderedQueryable<TEntity>)method.Invoke(null, [source, keySelector])!;
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}
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}
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}

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