добавила реализацию мастера

BeautySalon/BeautySalonDBModels/Implements/MasterDB.cs

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+using BeautySalonDBModels.Models;
+using Npgsql;
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace BeautySalonDBModels.Implements
+{
+    public class MasterDB : AbstractWorkWithStorage<Master>
+    {
+        public override void Add(Master master)
+        {
+            using var conn = GetConnection();
+            conn.Open();
+            using var cmd = new NpgsqlCommand("INSERT INTO masters (specialisation_id, fio) VALUES (@SpecialisationId, @FIO)", conn);
+            cmd.Parameters.AddWithValue("@SpecialisationId", master.SpecialisationId);
+            cmd.Parameters.AddWithValue("@FIO", master.FIO);
+            cmd.ExecuteNonQuery();
+        }
+
+        public override Master? GetObject(int id)
+        {
+            using var conn = GetConnection();
+            conn.Open();
+            using var cmd = new NpgsqlCommand("SELECT * FROM masters WHERE master_id = {id}", conn);
+            using var reader = cmd.ExecuteReader();
+            if (reader.Read())
+            {
+                return new Master
+                {
+                    MasterId = reader.GetInt32(0),
+                    SpecialisationId = reader.GetInt32(1),
+                    FIO = reader.GetString(2)
+                };
+            }
+            return null;
+        }
+
+        public override List<Master> GetObjects()
+        {
+            var masters = new List<Master>();
+            using var conn = GetConnection();
+            conn.Open();
+            using var cmd = new NpgsqlCommand("SELECT * FROM masters order by master_id", conn);
+            using var reader = cmd.ExecuteReader();
+            while (reader.Read())
+            {
+                masters.Add(new Master
+                {
+                    MasterId = reader.GetInt32(0),
+                    SpecialisationId = reader.GetInt32(1),
+                    FIO = reader.GetString(2)
+                });
+            }
+            return masters;
+        }
+
+        public override Master? Remove(int id)
+        {
+            var master = GetObject(id);
+            using var conn = GetConnection();
+            conn.Open();
+            using var cmd = new NpgsqlCommand("delete from masters where master_id = {id}", conn);
+            cmd.ExecuteNonQuery();
+            return master;
+        }
+
+        public override void Update(Master master)
+        {
+            using var conn = GetConnection();
+            conn.Open();
+            using var cmd = new NpgsqlCommand("update master set specialisation_id = {master.SpecialisationId}, " +
+                "fio = {master.FIO} where master_id = {master.MasterId}", conn);
+            cmd.ExecuteNonQuery();
+        }
+    }
+}