private QueryableMethodsVisitor PreEvaluateExpressionAndGetQueryableMethodsVisitor(ref Expression expression)
{
// replacing Count(predicate) with Where(predicate).Count()
expression = ScalarMethodsVisitor.Visit(expression);
// pre-executing everything, that can be executed locally
expression = SubtreeEvaluator.Value.EvaluateSubtree(expression);
// traversing the expression to find out the type of entities to be returned
var entityTypeExtractor = new EntityTypeExtractionVisitor();
entityTypeExtractor.Visit(expression);
if (entityTypeExtractor.TableEntityType == null)
{
throw new InvalidOperationException("Failed to extract the table entity type from the query");
}
var entityType = entityTypeExtractor.EntityType ?? entityTypeExtractor.TableEntityType;
// translating the query into set of conditions and converting the expression at the same time
// (all Queryable method calls will be replaced by a param of IQueryable<T> type)
var visitor = new QueryableMethodsVisitor(entityType, entityTypeExtractor.TableEntityType);
expression = visitor.Visit(expression);
visitor.TranslationResult.CustomizationHooks = this.CustomizationHooks;
return(visitor);
}
/// <summary>
/// Evaluates LINQ expression and queries DynamoDb table for results
/// </summary>
internal object ExecuteQuery(Expression expression)
{
// replacing Count(predicate) with Where(predicate).Count()
expression = ScalarMethodsVisitor.Visit(expression);
// pre-executing everything, that can be executed locally
expression = SubtreeEvaluator.Value.EvaluateSubtree(expression);
// traversing the expression to find out the type of entities to be returned
var entityTypeExtractor = new EntityTypeExtractionVisitor();
entityTypeExtractor.Visit(expression);
if (entityTypeExtractor.TableEntityType == null)
{
throw new InvalidOperationException("Failed to extract the table entity type from the query");
}
Type entityType = entityTypeExtractor.EntityType ?? entityTypeExtractor.TableEntityType;
// translating the query into set of conditions and converting the expression at the same time
// (all Queryable method calls will be replaced by a param of IQueryable<T> type)
var visitor = new QueryableMethodsVisitor(entityType, entityTypeExtractor.TableEntityType);
expression = visitor.Visit(expression);
// executing get/query/scan operation against DynamoDb table
var result = this._tableWrapper.LoadEntities(visitor.TranslationResult, entityType);
// trying to support other (mostly Enumerable's single-entity) operations
var enumerableResult = result as IEnumerable;
if (enumerableResult != null)
{
var queryableResult = enumerableResult.AsQueryable();
var lambda = Expression.Lambda(expression, visitor.EnumerableParameterExp);
// Here the default methods for IEnumerable<T> are called.
// This allows to support First(), Last(), Any() etc.
try
{
result = lambda.Compile().DynamicInvoke(queryableResult);
}
catch (TargetInvocationException ex)
{
throw ex.InnerException;
}
}
return(result);
}