diff --git a/README.md b/README.md
index 213fe1d..1c0d156 100644
--- a/README.md
+++ b/README.md
@@ -357,4 +357,4 @@ Current focus:
 - add stronger market metrics
 - expand pandas-based analytics workflows
 - improve dashboard usefulness without adding unnecessary chart noise
-- document metric definitions, assumptions and data-source behavior
\ No newline at end of file
+- document metric definitions, assumptions and data-source behavior
diff --git a/docs/forecast_research.md b/docs/forecast_research.md
index 5db8c1b..b0cc9b7 100644
--- a/docs/forecast_research.md
+++ b/docs/forecast_research.md
@@ -1,30 +1,40 @@
 # Research: First Forecasting Approach for Market Time Series
 
 ## 1. Realistic First Prediction Task for ARGUS
+
 A realistic first prediction task for ARGUS is **next-day exchange-rate movement** or **trend direction**. Predicting the exact next value (point forecast) is generally much harder and often less useful for trading/signal workflows than predicting the direction of the movement (up/down). A directional classification task serves as a simple, actionable signal for basic workflows.
 
 ## 2. Baseline Methods to Implement First
+
 Before jumping into complex models, the following baselines should be implemented to evaluate the added value of any machine learning model:
+
 - **Naive last-value forecast**: The prediction for the next period is exactly the value from the current period. This is surprisingly hard to beat in random walk-like financial time series.
 - **Moving average forecast**: A simple rolling average to predict the next value or determine trend direction.
 - **Simple linear regression**: To capture basic linear trends over a given historical window.
 
 ## 3. Libraries: NumPy, pandas, or scikit-learn?
+
 The first implementation should use **pandas** and **scikit-learn**:
+
 - **pandas**: Excellent for time-series manipulation, rolling windows, lagging features, and handling missing data.
 - **scikit-learn**: Offers robust implementations of simple models (e.g., Linear Regression, Logistic Regression for direction) and provides standardized metrics and cross-validation tools designed for time series (e.g., `TimeSeriesSplit`).
 
 ## 4. Evaluation Metrics
+
 For the initial approaches, we should focus on:
+
 - **Directional accuracy**: The percentage of times the model correctly predicts the direction of the price movement (up vs down). This is often more relevant than magnitude errors.
 - **MAE (Mean Absolute Error)**: If point forecasting is used, MAE is more robust to outliers than RMSE and provides a linear penalty for errors.
 - **RMSE (Root Mean Squared Error)**: Useful to penalize larger errors more heavily, but should be secondary to directional accuracy for basic signal generation.
 
 ## 5. Why is LSTM not the first implementation step?
+
 LSTMs are highly complex, require a large amount of well-structured data to train effectively without overfitting, and are notoriously difficult to tune. For financial time series, which suffer from low signal-to-noise ratios, an LSTM is likely to overfit the training data or collapse to predicting the last known value. Starting with an LSTM obscures whether the underlying data has any predictive power and sets a high barrier for debugging and infrastructure.
 
 ## 6. Prerequisites for an LSTM Ticket
+
 Before considering LSTMs or other deep learning approaches, the following must be established:
+
 - A reliable data ingestion and preprocessing pipeline.
 - Established baseline performance metrics (e.g., a naive model and a linear regression model) to compare against.
 - Sufficient historical data size.
@@ -32,7 +42,9 @@ Before considering LSTMs or other deep learning approaches, the following must b
 - Hardware/infrastructure to support longer training times and hyperparameter tuning.
 
 ## 7. Recommended First Implementation Approach
+
 **Recommendation**: Start with **directional trend prediction** (predicting whether the next value is higher or lower than the current value) using a simple **Logistic Regression** model via **scikit-learn**.
+
 - Use **pandas** to create basic lagged features (e.g., previous returns, moving averages).
 - Evaluate using **directional accuracy**.
 - Compare performance strictly against a **naive momentum** (predicting the trend continues) or **majority-class** baseline.
diff --git a/docs/research-data-sources b/docs/research-data-sources.md
similarity index 100%
rename from docs/research-data-sources
rename to docs/research-data-sources.md
diff --git a/docs/research-databases-and-storage.md b/docs/research-databases-and-storage.md
index 2061a29..484cd61 100644
--- a/docs/research-databases-and-storage.md
+++ b/docs/research-databases-and-storage.md
@@ -110,7 +110,7 @@ PostgreSQL should be introduced later when ARGUS moves toward a server-based or
 ## Local, Server and Cloud Options
 
 | Option | Meaning | Fit Now | Fit Later |
-|---|---|---:|---:|
+| --- | --- | ---: | ---: |
 | Local storage | Database runs locally inside or next to the project | High | High |
 | Server database | Database runs as a separate service, for example PostgreSQL | Medium | High |
 | Cloud storage/database | Managed storage or database in the cloud | Low | High |
@@ -203,7 +203,7 @@ ARGUS should not use a narrow `date | value` table as the main market-data model
 
 That would work for simple exchange rates, but it would become limiting once ARGUS adds stocks, ETFs, indices or broader market APIs.
 
-The first model should focus on three tables:
+The first model should focus on three related entities:
 
 ```text
 data_sources
@@ -211,11 +211,16 @@ instruments
 price_bars
 ```
 
+> [!NOTE]
+> The fields below describe the future database-oriented structure.
+> Technical fields such as `id`, `instrument_id`, `source_id`, `created_at` and `updated_at` are expected to appear in the database layer.
+> Internal Python models may reference related objects directly, for example `source` and `instrument`, before database IDs exist.
+
 ### data_sources
 
 Stores where data came from.
 
-Recommended first fields:
+Recommended first database fields:
 
 ```text
 id
@@ -226,13 +231,13 @@ created_at
 updated_at
 ```
 
-Example:
+Example internal/source records:
 
-| name | provider_kind | requires_api_key |
-|---|---|---:|
-| Frankfurter | fx_rates | false |
-| yfinance | market_prices | false |
-| FRED | macro_data | true |
+| name             | provider_kind | requires_api_key |
+| ---------------- | ------------- | ---------------: |
+| ExchangeRate API | fx_rates      |             true |
+| yfinance         | market_prices |            false |
+| FRED             | macro_data    |             true |
 
 ### instruments
 
@@ -246,7 +251,7 @@ Examples:
 * S&P 500
 * BTC-USD
 
-Recommended first fields:
+Recommended first database fields:
 
 ```text
 id
@@ -261,19 +266,19 @@ created_at
 updated_at
 ```
 
-Example:
+Example instrument records:
 
-| symbol | name | asset_class | currency | exchange | base_currency | quote_currency |
-|---|---|---|---|---|---|---|
-| EUR/USD | Euro / US Dollar | fx | null | null | EUR | USD |
-| AAPL | Apple Inc. | stock | USD | NASDAQ | null | null |
-| SPY | SPDR S&P 500 ETF | etf | USD | NYSE Arca | null | null |
+| symbol  | name             | asset_class | currency | exchange  | base_currency | quote_currency |
+| ------- | ---------------- | ----------- | -------- | --------- | ------------- | -------------- |
+| EUR/USD | Euro / US Dollar | fx          | null     | null      | EUR           | USD            |
+| AAPL    | Apple Inc.       | stock       | USD      | NASDAQ    | null          | null           |
+| SPY     | SPDR S&P 500 ETF | etf         | USD      | NYSE Arca | null          | null           |
 
 ### price_bars
 
 Stores historical market data in an OHLCV-ready structure.
 
-Recommended first fields:
+Recommended first database fields:
 
 ```text
 id
@@ -291,16 +296,16 @@ created_at
 updated_at
 ```
 
-For Frankfurter, the exchange rate can be stored in `close`.
+FX-style exchange-rate data can be represented as a price bar by storing the rate in `close`.
 
 The other OHLCV fields can stay empty until ARGUS uses data sources that provide them.
 
-Example:
+Example price bar records shown with joined source and instrument information for readability:
 
-| symbol | timestamp | timeframe | open | high | low | close | adjusted_close | volume |
-|---|---|---|---:|---:|---:|---:|---:|---:|
-| EUR/USD | 2024-01-02 | 1d | null | null | null | 1.095 | null | null |
-| AAPL | 2024-01-02 | 1d | 187.15 | 188.44 | 183.89 | 185.64 | 184.25 | 50200000 |
+| source   | instrument | timestamp  | timeframe |   open |   high |    low |  close | adjusted_close |   volume |
+| -------- | ---------- | ---------- | --------- | -----: | -----: | -----: | -----: | -------------: | -------: |
+| yfinance | EUR/USD    | 2024-01-02 | 1d        |   null |   null |   null |  1.095 |           null |     null |
+| yfinance | AAPL       | 2024-01-02 | 1d        | 187.15 | 188.44 | 183.89 | 185.64 |         184.25 | 50200000 |
 
 ---
 
@@ -332,7 +337,7 @@ Later sprints can expand the storage layer step by step.
 Possible later additions:
 
 | Future Area | Possible Additions |
-|---|---|
+| --- | --- |
 | Better source mapping | source-specific symbols, provider metadata |
 | Watchlists | user-selected instruments |
 | Reports | generated report metadata and history |
diff --git a/src/argus/domain/internal_models.py b/src/argus/domain/internal_models.py
new file mode 100644
index 0000000..3b7630e
--- /dev/null
+++ b/src/argus/domain/internal_models.py
@@ -0,0 +1,34 @@
+from dataclasses import dataclass
+from datetime import date
+
+
+@dataclass
+class DataSource:
+    name: str
+    provider_kind: str
+    requires_api_key: bool = False
+
+
+@dataclass
+class Instrument:
+    symbol: str
+    name: str
+    asset_class: str
+    currency: str | None = None
+    exchange: str | None = None
+    base_currency: str | None = None
+    quote_currency: str | None = None
+
+
+@dataclass
+class PriceBar:
+    source: DataSource
+    instrument: Instrument
+    timestamp: date
+    timeframe: str
+    close: float
+    open: float | None = None
+    high: float | None = None
+    low: float | None = None
+    adjusted_close: float | None = None
+    volume: float | None = None
diff --git a/tests/test_internal_models.py b/tests/test_internal_models.py
new file mode 100644
index 0000000..97df4c6
--- /dev/null
+++ b/tests/test_internal_models.py
@@ -0,0 +1,101 @@
+from argus.domain.internal_models import DataSource, Instrument, PriceBar
+from datetime import date
+
+
+def test_data_source_can_be_created() -> None:
+    source = DataSource(
+        name="yfinance",
+        provider_kind="fx_rates",
+    )
+
+    assert source.name == "yfinance"
+    assert source.provider_kind == "fx_rates"
+    assert source.requires_api_key is False
+
+
+def test_instrument_can_be_created() -> None:
+    instrument = Instrument(
+        symbol="EUR/USD",
+        name="Euro / US Dollar",
+        asset_class="fx",
+        base_currency="EUR",
+        quote_currency="USD",
+    )
+
+    assert instrument.symbol == "EUR/USD"
+    assert instrument.name == "Euro / US Dollar"
+    assert instrument.asset_class == "fx"
+    assert instrument.base_currency == "EUR"
+    assert instrument.quote_currency == "USD"
+    assert instrument.currency is None
+    assert instrument.exchange is None
+
+
+def test_rate_bar_can_be_created() -> None:
+    source = DataSource(
+        name="yfinance",
+        provider_kind="fx_rates",
+    )
+
+    instrument_rate = Instrument(
+        symbol="EUR/USD",
+        name="Euro / US Dollar",
+        asset_class="fx",
+        base_currency="EUR",
+        quote_currency="USD",
+    )
+
+    price_bar = PriceBar(
+        source=source,
+        instrument=instrument_rate,
+        timestamp=date(2026, 1, 1),
+        timeframe="1d",
+        close=1.89,
+    )
+
+    assert price_bar.source == source
+    assert price_bar.instrument == instrument_rate
+    assert price_bar.timestamp == date(2026, 1, 1)
+    assert price_bar.timeframe == "1d"
+    assert price_bar.close == 1.89
+    assert price_bar.open is None
+    assert price_bar.high is None
+    assert price_bar.low is None
+    assert price_bar.adjusted_close is None
+    assert price_bar.volume is None
+
+
+def test_stock_ohlcv_bar_can_be_created() -> None:
+    source = DataSource(
+        name="yfinance",
+        provider_kind="market_prices",
+    )
+
+    instrument = Instrument(
+        symbol="AAPL",
+        name="Apple Inc.",
+        asset_class="stock",
+        currency="USD",
+        exchange="NASDAQ",
+    )
+
+    price_bar = PriceBar(
+        source=source,
+        instrument=instrument,
+        timestamp=date(2026, 1, 1),
+        timeframe="1d",
+        open=187.15,
+        high=188.44,
+        low=183.89,
+        close=185.64,
+        adjusted_close=184.25,
+        volume=50_200_000,
+    )
+
+    assert price_bar.instrument.symbol == "AAPL"
+    assert price_bar.open == 187.15
+    assert price_bar.high == 188.44
+    assert price_bar.low == 183.89
+    assert price_bar.close == 185.64
+    assert price_bar.adjusted_close == 184.25
+    assert price_bar.volume == 50_200_000