Switch from position to strpos for text comparisons

Fixes npgsql#154

Both unit tests appear to only call into the `IndexOf` case in SqlBaseGenerator.cs but all three occurrences of `position` were changed just to be safe.

EF6.PG.Tests/EntityFrameworkBasicTests.cs

@@ -109,6 +109,46 @@ where p.Rating < 3
             }
         }
 
+        [Test]
+        public void Contains()
+        {
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                context.Blogs.Add(new Blog
+                {
+                    Name = "foo blog"
+                });
+                context.SaveChanges();
+            }
+
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                var searchTerms = new[] { "foo", "bar" };
+                var matches = context.Blogs.Count(b => searchTerms.Any(t => b.Name.Contains(t)));
+                Assert.AreEqual(1, matches);
+            }
+        }
+
+        [Test]
+        public void StartsWith()
+        {
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                context.Blogs.Add(new Blog
+                {
+                    Name = "foo blog"
+                });
+                context.SaveChanges();
+            }
+
+            using (var context = new BloggingContext(ConnectionString))
+            {
+                var searchTerms = new[] { "foo", "bar" };
+                var matches = context.Blogs.Count(b => searchTerms.Any(t => b.Name.StartsWith(t)));
+                Assert.AreEqual(1, matches);
+            }
+        }
+
         [Test]
         public void SelectWithWhere_Ef_TruncateTime()
         {

EF6.PG/SqlGenerators/SqlBaseGenerator.cs

@@ -935,7 +935,6 @@ VisitedExpression VisitFunction(EdmFunction function, IList<DbExpression> args,
         {
             if (function.NamespaceName == "Edm")
             {
-                VisitedExpression arg;
                 switch (function.Name)
                 {
                 // string functions
@@ -944,21 +943,17 @@ VisitedExpression VisitFunction(EdmFunction function, IList<DbExpression> args,
                     return OperatorExpression.Build(Operator.Concat, _useNewPrecedences, args[0].Accept(this), args[1].Accept(this));
                 case "Contains":
                     Debug.Assert(args.Count == 2);
-                    var contains = new FunctionExpression("position");
-                    arg = args[1].Accept(this);
-                    arg.Append(" in ");
-                    arg.Append(args[0].Accept(this));
-                    contains.AddArgument(arg);
+                    var contains = new FunctionExpression("strpos");
+                    contains.AddArgument(args[0].Accept(this));
+                    contains.AddArgument(args[1].Accept(this));
                     // if position returns zero, then contains is false
                     return OperatorExpression.Build(Operator.GreaterThan, _useNewPrecedences, contains, new LiteralExpression("0"));
                 // case "EndsWith": - depends on a reverse function to be able to implement with parameterized queries
                 case "IndexOf":
                     Debug.Assert(args.Count == 2);
-                    var indexOf = new FunctionExpression("position");
-                    arg = args[0].Accept(this);
-                    arg.Append(" in ");
-                    arg.Append(args[1].Accept(this));
-                    indexOf.AddArgument(arg);
+                    var indexOf = new FunctionExpression("strpos");
+                    indexOf.AddArgument(args[1].Accept(this));
+                    indexOf.AddArgument(args[0].Accept(this));
                     return indexOf;
                 case "Left":
                     Debug.Assert(args.Count == 2);
@@ -995,11 +990,9 @@ VisitedExpression VisitFunction(EdmFunction function, IList<DbExpression> args,
                     return Substring(args[0].Accept(this), args[1].Accept(this), args[2].Accept(this));
                 case "StartsWith":
                     Debug.Assert(args.Count == 2);
-                    var startsWith = new FunctionExpression("position");
-                    arg = args[1].Accept(this);
-                    arg.Append(" in ");
-                    arg.Append(args[0].Accept(this));
-                    startsWith.AddArgument(arg);
+                    var startsWith = new FunctionExpression("strpos");
+                    startsWith.AddArgument(args[0].Accept(this));
+                    startsWith.AddArgument(args[1].Accept(this));
                     return OperatorExpression.Build(Operator.Equals, _useNewPrecedences, startsWith, new LiteralExpression("1"));
                 case "ToLower":
                     return StringModifier("lower", args);